System and method for lithography process monitoring and control

ABSTRACT

In one aspect, the present invention is a technique of, and a system and sensor for measuring, inspecting, characterizing and/or evaluating optical lithographic equipment, methods, and/or materials used therewith, for example, photomasks. In one embodiment, the system, sensor and technique measures, collects and/or detects an aerial image produced or generated by the interaction between the photomask and lithographic equipment. An image sensor unit may measure, collect, sense and/or detect the aerial image in situ—that is, the aerial image at the wafer plane produced, in part, by a product-type photomask (i.e., a wafer having integrated circuits formed during the integrated circuit fabrication process) and/or by associated lithographic equipment used, or to be used, to manufacture of integrated circuits. In this way, the aerial image used, generated or produced to measure, inspect, characterize and/or evaluate the photomask is the same aerial image used, generated or produced during wafer exposure in integrated circuit manufacturing.  
     In another embodiment, the system, sensor and technique characterizes and/or evaluates the performance of the optical lithographic equipment, for example, the optical sub-system of such equipment. In this regard, in one embodiment, an image sensor unit measures, collects, senses and/or detects the aerial image produced or generated by the interaction between lithographic equipment and a photomask having a known, predetermined or fixed pattern (i.e., test mask). In this way, the system, sensor and technique collects, senses and/or detects the aerial image produced or generated by the test mask—lithographic equipment in order to inspect, evaluate and/or characterize the performance of the lithographic equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to: (1) U.S. ProvisionalApplication Serial No. 60/386,408, entitled “Complete OpticalLithography Inspection and Process Control”, filed Jun. 7, 2002; and (2)U.S. Provisional Application Serial No. 60/432,725, entitled “Method andApparatus for Aerial Imaging”, filed Dec. 12, 2002. The contents ofthese provisional applications are incorporated by reference herein intheir entirety.

BACKGROUND OF THE INVENTION

[0002] This invention relates to lithographic systems and techniquesthat are used in, for example, integrated circuit manufacturing; andmore particularly, in one aspect, to measure, inspect, characterizeand/or evaluate optical lithographic equipment, methods, and/orsub-systems related thereto (for example, the optical sub-systems andcontrol systems of the equipment as well as photomasks used therewith).

[0003] In the fabrication of integrated circuit, lithography is employedto “print” circuit patterns on a wafer (e.g., silicon or GaAssemiconductor substrate). Currently, optical lithography is thepredominant form of lithography used in volume integrated circuitmanufacturing. Optical lithography typically employs visible orultraviolet light to expose a given pattern (generally defined by thephotomask) on the resist that is disposed on a wafer to be transferredinto the substrate through resist development and subsequent processsteps, for example, etching, deposition and implantation

[0004] In optical lithography, the photomask (or mask), is first writtenusing electron-beam or laser-beam direct-write tools. The mask containscertain patterns and features that are used to create desired circuitpatterns on a wafer. The process of fabricating a complete integratedcircuit typically requires the use of many masks.

[0005] In the field of integrated circuit manufacturing, a commonlithographic tool used in projecting an image or pattern formed in aphotomask onto a wafer is known as a “stepper” or “scanner”. Withreference to FIG. 1, lithographic equipment 10 (for example, a stepper)may include mirror 12, light source 14 to generate light 16 at, forexample, an exposure wavelength λ_(o). The lithographic equipment 10 mayalso include illumination optics 18, projection optics 20, and a chuck22 upon which a wafer 24 is temporally secured, typically by way ofelectrostatic or vacuum forces, in a wafer plane. The mask 26 ispositioned and optically aligned to project an image of the circuitpattern to be duplicated onto wafer 24. The lithographic equipment 10may employ a variety of well known stepping, scanning or imagingtechniques to produce or replicate the mask pattern on wafer 24.

[0006] In general, there are three stages at which the integrity of thelithography process is measured, characterized or inspected. First, themask is inspected to determine whether the pattern on the maskaccurately represents the desired mask design. Second, the optics of thestepper (for example, light source 14, illumination optics 18, andprojection optics 20) are measured or characterized to confirm that theyare within acceptable guidelines. Third, the pattern “printed” or formedon the wafer or die (discrete pieces of the wafer) is inspected andanalyzed to determine or measure the quality of the fabrication process.

[0007] The photomasks are typically inspected first by the photomaskfabricator before providing them to an integrated circuit manufacturer,and then periodically by the integrated circuit manufacturer, forexample, during initial mask qualification and re-qualification. Thefabricator and manufacturer tend to use standalone equipment, forexample, tools made by KLA-Tencor (e.g., TeraStar series equipment) orApplied Materials (e.g., ARIS-100I equipment). This standaloneequipment, among other things, assesses the accuracy or integrity of thephotomask as well as its ability to produce an accurate representationof the circuit design onto the wafer or die, when used in conjunctionwith appropriate stepper optics and settings. While such inspectionequipment may provide an accurate representation of the photomask, ittends to be quite expensive and hence its use tends to be minimized.

[0008] Moreover, such inspection equipment often employs optical imagingsystems (or sub-systems) that are fundamentally different from that usedby the stepper in “printing” the image on the wafer during massproduction. For example, such standalone tools may include opticalimaging systems that employ wavelengths that are different from opticalimaging systems used in the mass production steppers. The responseand/or characteristics of photomask may depend upon the wavelength ofthe light used to measure or detect the mask (via, for example, anaerial image). Indeed, a photomask may exhibit defects in the productionstepper environment that may not be detectable in the standaloneinspection tool because, for example, detection of certain contaminantsdepends on wavelength. That is, certain contaminants may present seriousissues at the wavelength used during production but may be undetectableat the wavelength used during inspection.

[0009] The optics of the stepper are typically characterized by themanufacturer after the stepper is manufactured using grating andwavefront interference methods. The manufacturer may also employscanning electron microscopy (SEM) techniques to measure the patternsprinted, formed or projected on test wafers. In this regard, themanufacturer typically uses photomasks having specifically designed testpatterns. In this way, a resist pattern developed on a test wafer may bemeasured using SEM techniques and compared to a known, predetermined,fixed or expected pattern.

[0010] Due primarily to complexity of the inspection techniques, theinspection procedure of the stepper tends to require or consume anextended period of time, often days to complete, and thus represents anexpensive procedure for the integrated circuit manufacturer to carryout.

[0011] The integrated circuit manufacturer, however, may inspect andevaluate a stepper indirectly, using SEM inspection and analysis of thedeveloped resist image. Here again, due to the extended test time,inspection of the stepper is not performed very often, and, as a result,there are few samples and/or data to form a reliable measure of thestepper.

[0012] Conventional techniques to evaluate the final printed circuitpattern on the wafer or die tend to require examining the pattern formedon the wafer using SEM techniques. In this regard, the characterizationor verification of the accuracy and quality of the circuit patternpermits an indirect method of characterizing or verifying the mask andstepper (including optics), as well as the interactions between the maskand stepper. Because the final printed circuit pattern on the wafer ordie is formed after the resist development and may be after substratetreatment (for example, material etching or deposition), it may bedifficult to attribute, discriminate or isolate errors in the finalprinted circuit pattern to problems associated with the photomask, thestepper, or the resist deposition and/or the developing processes.Moreover, as with inspection of the optics of the stepper, inspectingthe final printed circuit pattern on the wafer or die using an SEM tendsto offer a limited number of samples upon which to detect, determine,and resolve any processing issues. This process may be labor intensiveand presents an extensive inspection and analysis time.

[0013] Thus, there is a need for a system and technique to overcome theshortcomings of one, some or all of the conventional systems andtechniques. In this regard, there is a need for an improved system andtechnique to inspect and characterize optical lithographic equipment,including the optical sub-systems, control systems and photomasks, thatare used in, for example, integrated circuit manufacturing.

[0014] In addition, there is a need for a system and technique ofphotomask inspection and characterization of in-situ or in a massproduction/fabrication environment. In this regard, there is a need fora system and technique to measure, sense, inspect, detect, captureand/or evaluate the aerial image of a photomask in situ—that is, in themass production environment using the lithographic production equipmentof that environment. In this way, the errors may be isolated andattributed to a given aspect of the process or system. Indeed, thecauses of errors in a final printed circuit pattern may be isolated,characterized and/or measured (in, for example, the photomask, stepper,and/or resist developing process) so that appropriate correctivemeasures may be determined efficiently, rapidly and in a cost-effectivemanner. Thus, there is a need for a system and technique that permitserrors in the lithographic fabrication process to be attributed orisolated to certain methods or equipment (for example, the photomask oroptical sub-system) in order to facilitate appropriate and/or efficientcorrection of such errors in the final printed circuit pattern andthereby enhance or improve the quality, yield and cost of integratedcircuits.

[0015] Further, there is a need for an improved lithographic imageevaluation technique and system that overcomes one, some or all of theconventional systems and techniques. In this regard, there is a need fora system and technique to more thoroughly, quickly and/or more oftenevaluate and calibrate lithographic imaging systems, for example,steppers, in an efficient and cost-effective manner. In this way, thequality, yield and cost of integrated circuits may be improved.

SUMMARY OF THE INVENTION

[0016] There are many inventions described herein. In a first principalaspect, the present invention is an image sensor unit, for use with ahighly precise moveable platform. The image sensor unit of this aspectof the invention includes a substrate having a wafer-shaped profile orform factor that may allow automated handling of image sensor unit inthe same manner as a product-type wafer. The image sensor unit furtherincludes a sensor array (for example, charge coupled, CMOS or photodiodedevices) disposed on the substrate.

[0017] The sensor array includes a plurality of sensor cells whereineach sensor cell includes an active area to sense light of apredetermined wavelength that is incident thereon. The sensor array alsoincludes a film, disposed over the active areas of sensor cells andcomprised of a material that impedes passage of light of thepredetermined wavelength. The film includes a plurality of aperturesthat are arranged such that at least one aperture overlies an activearea of a corresponding sensor cell to expose a portion of the activearea and wherein light of the predetermined wavelength is capable ofbeing sensed by the portion of the active area that is exposed by thecorresponding aperture.

[0018] In one embodiment of this aspect of the invention, the imagesensor unit may include a transparent medium, having a predeterminedrefractive index, disposed on the sensor array. In another embodiment,the image sensor unit may include photon-conversion material disposedover and/or within the sensor array. The photo-conversion material maybe disposed between the film and the plurality of sensors.

[0019] In another embodiment, the image sensor unit may includecommunications circuitry disposed on the substrate. The communicationscircuitry may employ wired, wireless and/or optical techniques. In oneembodiment, the communications circuitry outputs data from the sensorarray, using wired and/or wireless techniques, during collection ofimage data by the sensor array.

[0020] In another embodiment, the image sensor unit may include at leastone battery, disposed on the wafer-shaped substrate or within a cavityin the wafer-shaped substrate. The battery may be rechargeable and mayprovide electrical power to the sensor array and/or the communicationscircuitry.

[0021] In another embodiment, the image sensor unit may also includedata storage circuitry and data compression circuitry. In thisembodiment, the data storage circuitry is coupled to the sensor array toreceive and store the data from the sensor array. The data compressioncircuitry is coupled to the data storage circuitry to compress the data.

[0022] In another principal aspect, the present invention is an imagesensor unit, for use with a highly precise moveable platform, whichincludes a wafer-shaped substrate and a sensor array, integrated intothe substrate. The sensor array includes a plurality of sensor cells(for example, charge coupled devices, CMOS devices or photodiodes)wherein each sensor cell includes an active area to sense light of apredetermined wavelength that is incident thereon. The sensor array alsoincludes a film, disposed over the plurality of active areas of thesensor cells and comprised of a material that impedes passage of lightof the predetermined wavelength. The film includes a plurality ofapertures that are arranged such that an aperture of the plurality ofapertures overlies an active area of a corresponding sensor cell toexpose a portion of the active area. In this way, light of thepredetermined wavelength is capable of being sensed by the portion ofthe active area that is exposed by the corresponding aperture.

[0023] In one embodiment of this aspect of the present invention, theimage sensor unit may include communications circuitry disposed on thesubstrate. The communications circuitry may employ wired, wirelessand/or optical techniques. In one embodiment, the communicationscircuitry outputs data from the sensor array, using wireless techniques,during collection of image data by the sensor array.

[0024] In another embodiment, the image sensor unit may include at leastone battery, disposed on the wafer-shaped substrate or within a cavityin the wafer-shaped substrate. The battery may be rechargeable.

[0025] In yet another embodiment, the image sensor unit may also includedata storage circuitry and data compression circuitry. In thisembodiment, the data storage circuitry is coupled to the sensor array toreceive and store the data from the sensor array. The data compressioncircuitry is coupled to the data storage circuitry to compress the data.

[0026] The image sensor unit may include photon-conversion materialdisposed over and/or within the sensor array. In another embodiment, thephoto-conversion material is disposed between the film and the pluralityof sensors.

[0027] In yet another principal aspect, the present invention is asystem to collect image data which is representative of an aerial imageof a mask (for example, a product-type or test mask) that is projectedon a wafer plane. The system includes an optical system to produce theimage of the mask on the wafer plane, a moveable platform and an imagesensor unit, disposed on the moveable platform, to collect image datawhich is representative of the aerial image of the mask.

[0028] The image sensor unit includes a wafer-shaped substrate and asensor array. The sensor array is disposed on or in the wafer-shapedsubstrate, such that when position on the moveable platform, the sensorarray is disposed in the wafer plane.

[0029] The sensor array includes a plurality of sensor cells whereineach sensor cell includes an active area to sense light of apredetermined wavelength that is incident thereon. The sensor arrayfurther includes a film, disposed over the active areas of the sensorcells. The film is comprised of a material that impedes passage of lightof the predetermined wavelength and includes a plurality of apertureswhich are arranged such that an aperture of the plurality of aperturesoverlies a corresponding active area of a corresponding sensor cell toexpose a portion of the active area. In this way, the light of thepredetermined wavelength is capable of being sensed by the portion ofthe active area that is exposed by the corresponding aperture.

[0030] In one embodiment of this aspect of the present invention, theimage sensor unit may include communications circuitry disposed on thesubstrate. The communications circuitry may employ wired, wirelessand/or optical techniques. In one embodiment, the communicationscircuitry outputs data from the sensor array, using wired and/orwireless techniques, during collection of image data by the sensorarray.

[0031] In another embodiment, the image sensor unit may include a dataprocessing unit and/or at least one battery (for example, arechargeable-type), disposed on, or within a cavity in, the wafer-shapedsubstrate, to provide electrical power to the sensor array and/or thecommunications circuitry. The data processing unit may be configured toreceive the image data which is representative of the aerial image.

[0032] In one embodiment, the moveable platform may move in first andsecond directions to a plurality of discrete locations wherein at eachdiscrete location, the sensor cells sample the light incident on theexposed portion of the active area. The data processing unit may use thedata to generate the aerial image.

[0033] The distance between the plurality of discrete locations in thefirst direction may be less than or equal to the width of the apertures.Further, the distance between the plurality of discrete locations in thesecond direction may be less than or equal to the width of theapertures. In one embodiment, the processing unit interleaves the imagedata to generate the aerial image.

[0034] In one embodiment, the image sensor unit collects data which isrepresentative of the aerial image in a raster-type manner. In anotherembodiment, the image sensor unit collects image data which isrepresentative of the aerial image in a vector-type manner.

[0035] In another aspect, the present invention is an image sensor unitthat may be employed to collect image data which is representative of anaerial image of a mask (for example, a product-type mask) that isprojected on a wafer plane by a lithographic unit. The image sensor unitof this aspect of the invention includes a sensor array which isdisposed in the moveable platform of the lithographic unit and capableof being located in the wafer plane. The sensor array (for example, acharge coupled, CMOS or photodiode device) includes a plurality ofsensor cells wherein each sensor cell includes an active area to senselight of a predetermined wavelength that is incident thereon. The sensorarray also includes a film, disposed over the active areas of theplurality of sensor cells and comprised of a material that impedespassage of light of the predetermined wavelength. The film includes aplurality of apertures which are arranged such that an aperture of theplurality of apertures overlies a corresponding active area of acorresponding sensor cell to expose a portion of the active area so thatlight of the predetermined wavelength is capable of being sensed by theportion of the active area that is exposed by the correspondingaperture.

[0036] In one embodiment, the sensor array is capable of being movedbetween a plurality of discrete locations in first and second directionswhile disposed on the moveable platform. The sensor cells sample thelight incident on the exposed portion of the active area at eachdiscrete location. The distance between the plurality of discretelocations in the first direction may be less than or equal to the widthof the apertures. Further, the distance between the plurality ofdiscrete locations in the second direction may be less than or equal tothe width of the apertures. In one embodiment, the processing unitinterleaves the image data to generate the aerial image.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In the course of the detailed description to follow, referencewill be made to the attached drawings. These drawings show differentaspects of the present invention and, where appropriate, referencenumerals illustrating like structures, components, materials and/orelements in different figures are labeled similarly. It is understoodthat various combinations of the structures, components, materialsand/or elements, other than those specifically shown, are contemplatedand are within the scope of the present invention.

[0038]FIG. 1 is a block diagram representation of a conventional opticalstepper used in the fabrication process of an integrated circuit;

[0039]FIG. 2 is a block diagram representation of a system according toat least one embodiment of the present invention;

[0040] FIGS. 3A-C are block diagram representations of a wafer-shapedimage sensor unit, according to certain embodiments of the presentinvention;

[0041]FIG. 3D is block diagram representation of an image sensor unitintegrated on or in a chuck of lithographic equipment, according to oneembodiment of the present invention;

[0042]FIG. 4 is a block diagram representation of a sensor arrayaccording to one embodiment of the present invention;

[0043]FIG. 5 is a two-dimensional (top view) schematic representation ofthe sensor array, in conjunction with a selectively patterned, shapedand/or etched opaque film, according to one embodiment of the presentinvention;

[0044]FIGS. 6A and 6B are cross-sectional views of the sensor array, inconjunction with a selectively patterned, shaped and/or etched opaquefilm, according to certain embodiments of FIG. 5;

[0045]FIGS. 7A and 7B are cross-sectional views of a sensor array, inconjunction with a selectively patterned, shaped and/or etched opaquefilm as well as a transparent medium of a predetermined refractiveindex, according to several embodiments of the present invention;

[0046]FIGS. 8A, 8B, 9A-D, 10 and 11A-C are cross-sectional views of thesensor cells and/or sensor array according to other embodiments of thepresent invention;

[0047] FIGS. 12A-C and 13A-E are representations of the image collectiontechniques, in relation to the sensor array, according to certainaspects of the present invention;

[0048] FIGS. 14A-C are representations of image collection techniques,in relation to the sensor array, according to certain aspects of thepresent invention;

[0049]FIGS. 15A and 15B are exemplary representations of an imagecollection technique, in relation to a sensor array that is smaller thanthe aerial image to be collected, according to certain aspects of thepresent invention;

[0050]FIG. 16 is a representation of an image collection technique, inrelation to the sensor array, according to another aspect of the presentinvention; and

[0051]FIG. 17 is a representation of the image collection technique, inrelation to the sensor array, according to another aspect of the presentinvention;

[0052]FIGS. 18A and 18B are block diagram representations of a sensorarray having a plurality of sub-sensor arrays; and

[0053]FIGS. 19 and 20 are exemplary representations of image collectiontechniques, in relation to a sensor array that is smaller than theaerial image to be collected, according to certain aspects of thepresent invention.

DETAILED DESCRIPTION

[0054] There are many inventions described herein. In one aspect, thepresent invention is directed to a technique of, and system formeasuring, inspecting, characterizing and/or evaluating opticallithographic equipment, methods, and/or materials used therewith, forexample, photomasks. In this regard, the present invention is a system,sensor and technique to sample, measure, collect and/or detect an aerialimage produced or generated by the interaction between the photomask andlithographic equipment. An image sensor unit may be employed to sample,measure, collect and/or detect the aerial image of a product-typephotomask (i.e., a photomask that is used in the production ofintegrated circuits on product wafers) in situ—that is, the aerial imageat the wafer plane produced by the interaction between the photomask andthe lithographic equipment that are used (or to be used) duringmanufacture of integrated circuits. In this way, the aerial image used,generated or produced to measure, inspect, characterize and/or evaluatethe photomask is the same aerial image used, generated or producedduring wafer exposure in integrated circuit manufacturing.

[0055] In another aspect, the present invention is a technique of, andsystem and sensor for measuring, inspecting, characterizing, sensingand/or evaluating the performance of the optical lithographic equipment,for example, the optical sub-system of such equipment. In this regard,in one embodiment, an image sensor unit measures, collects, senses,and/or detects the aerial image produced or generated by the interactionbetween lithographic equipment and a photomask having a known,predetermined or fixed pattern (i.e., test mask). The sensor unitsenses, collects and/or detects the aerial image produced or generatedby the test mask—lithographic equipment in order to measure, inspect,and/or characterize the performance of the optical sub-system of thelithographic equipment.

[0056] In another aspect, the present invention is a technique of, andimaging system and sensor for generating or producing the same orsubstantially the same aerial image (or producing, sampling, collectingand/or detecting information relating to), with the same orsubstantially the same spatial resolution, as certain lithographicequipment (for example a particular stepper system having a given set ofparameters, features or attributes). Here, the imaging system emulatesthat lithographic equipment. As such, the imaging system includes aprecision mechanical movement stage having the same or substantially thesame mechanical precision and controllability as lithographic equipment.The imaging system may be employed as a standalone aerial imagemonitoring tool that may be used in the reviewing of the aerial image ofa mask under the predetermined optic sub-system.

[0057] This “standalone tool” may be designed and fabricated to have oneor more of the following parameters that is/are the same orsubstantially the same as one or more of the following features orparameters of the lithographic equipment: (1) wavelength of light; (2)characteristics of light source (e.g., excimer laser); (3) illuminationsystem including partial coherence; and (4) numerical aperture (NA). Inthis way, the differences in the aerial image collected, sampled,produced, and/or measured using the imaging system and the sensor, inrelation to the lithographic equipment, may be minimized and/or reduced.Moreover, the standalone tool of this aspect of the invention may be a“mini-stepper” (which has a much smaller field than a productionstepper, but otherwise substantially the same imaging properties).

[0058] In certain embodiments of the invention, the image sensor unitmay be disposed on, or integrated in a wafer-shaped platform orsubstrate. The sensor of this embodiment includes a profile (forexample, height and shape, and flatness of the sensing cells of thesensor) that facilitates implementation in lithographic equipment muchlike a product-type wafer and maintains a stage position along theoptical axis similar to, or substantially similar to, that of a productwafer. In this regard, the wafer-shaped sensor may be secured to thechuck and reside in the wafer plane in a manner similar to aproduct-type wafer. Moreover, the wafer-shaped platform may permitautomated handling by a robotic loader of the lithographic equipment. Inaddition, electrical power may be supplied to the image sensor unit by abattery (rechargeable or otherwise) and/or the lithographic equipment;and data/command transmission may be accomplished using wired, wirelessand/or optical techniques.

[0059] In other embodiments, the image sensor unit may be disposed in,or integral with a subsystem, for instance, the wafer chuck of thelithographic equipment. In this way, the image sensor unit need not beloaded into the lithographic equipment but may be positioned in thewafer plane during inspection, characterization and/or evaluation of aphotomask and/or the optical system of the lithographic equipment. Theelectrical power may be provided to the image sensor unit from thelithographic equipment. Moreover, the image sensor unit mayreceive/provide data and commands using wired, wireless, or opticalcommunications.

[0060] Thus, in one aspect, the present invention measures, inspects,characterizes and/or evaluates photomasks directly, and in the actualproduction environment in which the photomasks are used during themanufacture of integrated circuits (i.e., employing the same orsubstantially the same lithographic equipment used during integratedcircuit fabrication or production). In another aspect, the presentinvention measures, inspects, characterizes and/or evaluates the opticalsystem of lithographic equipment using a photomask having a known,predetermined or fixed pattern.

[0061] In another aspect, the present invention is an image sensor thatincludes an array of sensor cells wherein each sensor cell includes anenhanced, limited or restricted spatial resolution. In this regard, eachsensor cell has “effective” active or photon sensitive area that is lessthan or substantially less (i.e., greater than 50%) than the active orphoton sensitive area of the sensor cell. The image sensor unit of thisaspect of the present invention may be employed on or in a highlyprecise spatial positioning and/or moveable platform, for example thestage of a stepper. In this way, the image sensor may measure, detect,sense, collect and/or sample an aerial image projected or incidentthereon. The data measured, detected, sensed, collected and/or sampledby all sensor cells may be combined to construct the aerial image, or todeduce relevant information directly without constructing the aerialimage. The data may also be measured, detected, sensed, collected and/orsampled using vectoring (absolute coordinates) techniques or vectorscanning techniques.

[0062] With reference to FIG. 2, in one embodiment, aerial image sensingsystem 100 of the present invention includes lithographic equipment 10(for example, a stepper), image sensor unit 102, andprocessor/controller 104, for example, a computer and/or data or imageprocessing unit. The lithographic equipment 10 is similar to thatdescribed above with respect to FIG. 1. That is, lithographic equipment10 may include mirror 12, light source 14 to generate light 16 at, forexample, an exposure wavelength λ_(o), illumination optics 18,projection optics 20, and a chuck 22. The chuck 22 secures sensor unit102 in a fixed location, using, for example, electrostatic or vacuumforces.

[0063] The optics of lithographic equipment 10 (for example, lightsource 14, illumination optics 18, and projection optics 20) interactswith mask 26 to project an aerial image onto image sensor unit 102. Inone embodiment, photomask 26 may be a product-type mask; that is, aphotomask used to form circuits during integrated circuit fabrication.As such, photomask 26, in one embodiment, contains the pattern to bereplicated or printed on a wafer that ultimately contains the circuitdesign (or a portion thereof of the integrated circuit. In thisembodiment, image sensor unit 102 may be employed to evaluate theinteraction between photomask 26 and lithographic equipment 10 (whetherproduction or non-production equipment) as well as characterize theperformance of lithographic equipment 10.

[0064] In another embodiment, mask 26 may be a test mask that is used toinspect, characterize and/or evaluate the optical characteristics orresponse of lithographic equipment 10. In this regard, mask 26 mayinclude a fixed, predetermined and/or known pattern against which theaerial image collected, sensed, sampled, measured and/or detected byimage sensor unit 102 will be evaluated, measured, and/or compared. Inthis way, any errors or discrepancies in the aerial images may beisolated or attributed to the optical system of lithographic equipment10 and the performance of that system may be evaluated or characterized.

[0065] With continued reference to FIG. 2, image sensor unit 102collects, measures, senses and/or detects the aerial image produced orgenerated by lithographic equipment 10 in conjunction with photomask 26.The image sensor unit 102 provides image data, which is representativeof the aerial image, to processor/controller 104. Theprocessor/controller 104, in response, evaluates and/or analyzes thatdata to inspect, characterize and/or evaluate photomask 26 and/orlithographic equipment 10 (or sub-systems thereof, for example, theoptical sub-system). In this regard, processor/controller implementsdata processing and analysis algorithms to process the data from imagesensor unit 102 to reconstruct a full or partial aerial image, or toextract desired information directly without reconstructing a full orpartial aerial image. Such image processing may involve deconvolution orother techniques familiar to those skilled in the art.

[0066] In addition, processor/controller 104 may use the data fromsensor unit 102, to perform and evaluate critical dimensionmeasurements, and/or conduct defect inspection, for example, bycomparing the measured aerial image to pattern design database, or dodie-to-die inspection if there are multiple dice on the same mask. Theprocessor/controller 104 may also implement algorithms that conduct orperform resist modeling and/or integrated circuit yield analyses.

[0067] The processor/controller 104 may be employed as a control oroperator console and data/image processing device. Theprocessor/controller 104 may store algorithms and software that processthe data representative of the aerial image (received from image sensorunit 102), extract information, manage data storage, and/or interfacewith users/operators. The processor/controller 104 may be located nearor next to lithographic equipment 10 or in another locale, which isremote from lithographic equipment 10.

[0068] The processor/controller 104 may also provide and/or applyappropriate corrective measures to lithographic equipment 10 in order toenhance or improve the performance or operation of lithographicequipment 10 and/or enhance or improve the interaction between mask 26and lithographic equipment 10. In this way, the quality, yield and costof integrated circuits fabricated using lithographic equipment 10 and/ormask 26 may be improved or enhanced.

[0069] It should be noted that processor/controller 104 may be astand-alone unit, as illustrated in FIG. 2, or partially or whollyintegrated in lithographic equipment 10. In this regard, suitablecircuitry in lithographic equipment 10 may perform, execute and/oraccomplish the functions and/or operations of processor/controller 104(for example, evaluation and/or analysis of the data representative ofthe aerial image collected, measured, sensed and/or detected at thewafer plane). Thus, in one embodiment, the inspection, characterizationand/or evaluation circuitry/electronics may be partially or whollyintegrated into lithographic equipment 10 and, as such, this “integratedsystem” may determine, assess, apply and/or implement appropriatecorrective measures to enhance or improve its operation and therebyimprove or enhance the quality, yield and cost of integrated circuitsmanufactured therein.

[0070] It should be further noted that processor/controller 104 may alsobe partially or wholly integrated in, or on, image sensor unit 102. Inthis regard, some or all of the functions and operations to be performedby processor/controller 104 may be performed, executed and/oraccomplished by suitable circuitry in, or on image sensor unit 102. Assuch, the collection and analysis of data representative of the aerialimage may be less cumbersome in that a bus may be integrated and/orfabricated on or within image sensor unit 102 to facilitatecommunication of data and commands to/from the circuitry used tomeasure, detect and/or sense the aerial image and the circuitry used toevaluate and/or analyze the data representative of the aerial image.

[0071] It should be noted that, in at least one embodiment,processor/controller 104 may interact with multiple sensor units 102and/or multiple lithographic equipment 10.

[0072] With reference to FIG. 3A, in one embodiment, image sensor unit102 includes sensor array 106, controller 108, batteries 110, datacompression circuitry 112, transmitter circuitry 114, andtransmitter/receiver circuitry 116. The image sensor unit 102 may beformed in or on substrate 118. The substrate 118, and, in particular,image sensor 102, may include a size and shape that facilitates imagesensor 102 being accepted by, or installed or mounted on chuck 22 oflithographic equipment 10 in a manner that is similar to that of aproduct wafer. As such, image sensor unit 102, and in particular sensorarray 106, may reside in the wafer plane so the aerial image measured,collected, sensed or detected is the same (or substantially the same) asthe aerial image projected on a product wafer by the interaction of mask26 and lithographic equipment 10.

[0073] Moreover, the wafer-shaped size and shape of image sensor unit102 may permit normal and/or typical operation of lithographic equipment10. In those instances where lithographic equipment 10 includesautomated loading of wafers, the wafer-shaped profile or form factor ofimage sensor 102 may allow automated handling of image sensor unit 102in the same manner as a product-type wafer. Indeed, in a preferredembodiment, the wafer-shaped size and shape of image sensor 102 includesa physical form factor that permits the wafer handling apparatus(whether automated or partially or fully manual) of lithographicequipment 10 to manipulate the image sensor 102 without significantmodifications to the wafer handling apparatus. In this way, the“down-time” of lithographic equipment 10, due to inspection and/orcharacterization of the interaction between mask 26 and lithographicequipment 10, may be minimized.

[0074] The sensor array 106 may be a plurality of photo or photonsensitive detectors or sensor cells that measure, sense, detect and/orcollect incident energy or radiation, for example, incident visible orultraviolet light (for example, deep ultraviolet light). With referenceto FIG. 4, in one embodiment, sensor array 106 includes a plurality ofsensor cells 200 a ₁-200 a ₈, 200 b ₁-200 b ₈, 200 c ₁-200 c ₈, 200 d ₁to 200 d ₈, 200 e ₁-200 e ₈, 200 f ₁-200 f ₈, 200 g ₁-200 g ₈, and 200 h₁-200 h ₈, arranged or configured in a two dimensional array. The sensorcells 200 a _(x)-200 h _(x) (x=1 to 8) of sensor array 106 may becomprised of light or radiation sensing semiconductor devices, forexample, charge coupled devices (CCDs), CMOS sensor cells and/or photodiodes.

[0075] With continued reference to FIG. 4, sensor cells 200 a _(x)-200 h_(x) (x=1 to 8) include active area 202 a _(x)-202 h _(x) (x=1 to 8),respectively. The active area 202 a _(x)-202 h _(x) is that portion orarea of sensor cells 200 a _(x)-200 h _(x) (x=1 to 8), respectively,which is sensitive to the energy or radiation incident thereon. Thedimensions of active areas 202 a _(x)-202 h _(x) (x=1 to 8) may impactthe spatial resolution of the aerial image.

[0076] In those instances where the dimensions of active areas 202 a_(x)-202 h _(x) (x=1 to 8) are too large to provide a desired orrequired spatial resolution, it may be necessary to limit, restrictand/or reduce the photo-sensitive area of sensor cells 200 a _(x)-200 h_(x) (x=1 to 8). With reference to FIGS. 5 and 6, sensor array 106, inone embodiment, may include a patterned opaque film 204 that impedes,obstructs, absorbs and/or blocks passage of photons or light of a givenwavelength (that is, at the wavelength to be measured, sensed ordetected by sensor cells 200 a _(x)-200 h _(x), x=1 to 8).

[0077] The opaque film 204 includes apertures 206 a _(x)-206 h _(x) (x=1to 8). The apertures 206 a _(x)-206 h _(x) (x=1 to 8) are configured orarranged to overlie a respective one of active area 202 a _(x)-202 h_(x) (x=1 to 8). In this way, opaque film 204 overlies sensor cells 200a _(x)-200 h _(x) (x=1 to 8) to partially cover active areas 202 a_(x)-202 h _(x) (x=1 to 8) and thereby limit the photo-sensitive area ofactive area 202 a _(x)-202 h _(x) (x=1 to 8) to the portion(s)effectively exposed by apertures 206 a _(x)-206 h _(x) (x=1 to 8). Theportion of active area 202 a _(x)-202 h _(x) (x=1 to 8) that is coveredby film 204 does not measure, sense, detect and/or collect incidentenergy or radiation or is substantially unaffected by such energy orradiation. As such, the spatial resolution of the energy measured bysensor cells 200 a _(x)-200 h _(x) (x=1 to 8) is enhanced or improvedbecause the portion or area of the sensor cell that is effectivelyexposed to, and/or measures, senses, detects and/or collects energy orradiation is limited or restricted.

[0078] In certain embodiments, it may be advantageous to selectivelypattern opaque film 204 to include apertures 206 a _(x)-206 h _(x) (x=1to 8) that are located or positioned in, or near, the center of activeareas 202 a _(x)-202 h _(x) (x=1 to 8). In this way, a significantnumber of photons that enter apertures 206 a _(x)-206 h _(x) (x=1 to 8)may be collected, measured, sensed and/or detected by the underlyingactive areas 202 a _(x)-202 h _(x) (x=1 to 8), respectively, regardlessof scattering caused or induced by apertures 206 a _(x)-206 h _(x) (x=1to 8). In addition, locating or positioning apertures 206 a _(x)-206 h_(x) (x=1 to 8) at or near the center of active areas 202 a _(x)-202 h_(x) (x=1 to 8) may ease alignment constraints during fabrication ofopaque film 204 and apertures 206 a _(x)-206 h _(x) (x=1 to 8).

[0079] The opaque film 204 may be any material that may be deposited,grown and/or formed on or in sensor cells 200, and patterned, shapedand/or etched such that active areas 202 receive, measure, collectphotons from a smaller, limited and/or restricted area or region(substantially or proportionally equal to the area of apertures 206relative to the entire active area). The opaque film 204 may be a metalor other material (for example, ploy-silicon or amorphous-silicon) thateffectively blocks the light/radiation at the wavelength of concern. Forexample, opaque film 204 may be a film, such as tungsten, silicon,platinum, aluminum, chromium, copper, gold, silver, or an oxide (forexample, Ta₂O₅, tantalum-pentoxide) of a sufficient thickness to alter,modify, impede, obstruct, absorbs and/or block photons or light (of atleast a given wavelength) from being measured, sensed and/or detected bythat portion of active area 202. In one embodiment, opaque film 204 maybe tungsten of a thickness in the range of 100 nm. Indeed, any materialthat (wholly, substantially or effectively) alter or modifies photons orlight (of at least a given wavelength) from being measured, sensedand/or detected by a certain portion of active areas 202, and/or anymaterial that impedes, obstructs, absorbs and/or blocks passage of thewavelength of the photons to be measured, sensed or detected by sensorcells 200, whether now known or later developed, is intended to bewithin the scope of the present invention.

[0080] The opaque film 204 should be of sufficient thickness to wholly,partially or effectively impede, obstruct, absorb and/or block passageof the wavelength of the photons to be measured, sensed or detected bysensor cells 200. In certain embodiments, the thicker film 204, the moreeffective film 204 may be in impeding, obstructing, absorbing and/orblocking passage of at least the wavelength of interest. However, incertain instances, a thicker film may present more difficulties informing apertures 206 in film 204.

[0081] Moreover, a thicker film 204 may also present a higher aspectratio (i.e., the ratio between aperture wall height and aperturediameter). A higher aspect ratio may allow less light impinging uponsensor cells 200 to pass through apertures 206 as well as cause anaperture “response” function that is more complex than a summation ofthe photon energy impinging on the inlet of aperture 206. In thisregard, a higher aspect ratio may cause opaque film 204 to block obliquerays (due, for example, to high NA optics) more than the film blocksrays having a straight angle. Further, higher aspect ratio increases theaperture response function's dependence on the process variation, e.g.,for the same amount of sidewall angle variation between apertures, theaperture response changes more when the aspect ratio is higher. As such,in one embodiment, the thickness of opaque film 204 is selected to be aminimum but also sufficient to block, obstruct, and/or absorb theradiation to be measured, sensed or detected by sensor cells 200.

[0082] In that regard, where the total light collecting area under theopaque film is A₁, and the aperture area is A₂, and assuming the signalfrom the aperture to be at least N times larger than the total signalfrom the blocked area (N can be called as the signal-to-noise-ratio,i.e. SNR, of the blocking), in one embodiment the attenuation of theopaque film to the light may be characterized as:

D=N*(A ₁ /A ₂)

[0083] As such, where N=1000, A1 is 5 μm×5 μm, and A2 is 100 nmdiameter, the attenuation factor D will need to be 3.2e⁶.

[0084] The attenuation may be computed or characterized using thefollowing steps:

[0085] (1) Penetration depth of the light in the film is: d=λ/(2*π*k),where λ is the wavelength of light, and k is the imaginary part of therefractive index, which depends on the wavelength and material, and canbe looked up in material handbooks (see, for example, “Handbook ofOptical Constants of Solids”, Editor Edward D. Palik, Volume 1, 2, 3).The k value is usually in the range of 1.5 to 4.0. The larger the k, thesmaller the penetration depth and the better blocking.

[0086] (2) With the known penetration depth, the attenuation afterthickness of T is then:

D=e^(T/d)

[0087] Hence, for a chosen opaque material and any specific wavelength,the thickness T may be computed to achieve the required or desiredattenuation factor D. To give an order of magnitude, using, for example,tungsten or poly-silicon as the material for opaque film 204, for 193 nmwavelength, and using the area ratio used in the above example, the filmthickness may be around 100 nm.

[0088] In one embodiment, as illustrated in FIG. 6B, opaque film 204includes a step configuration around the proximity of apertures 206.Where opaque film 204 is fabricated from a metal material, a first layeror lower step may be deposited or formed and, at the same time orshortly thereafter, a second layer or upper step may be deposited orformed between apertures 206. In this embodiment, the configuration ofopaque film 204 may effectively change the area ratio (A₁/A₂) to thearea ratio between the surface of the lower layer of film 204 andaperture 206. In one embodiment, this ratio may be less than 5.

[0089] Notably, in the example above, this embodiment may reduce therequirement on the attenuation ratio (D) by a factor of about 500, whichcould translate to about 40% reduction in the required aperture aspectratio (which may be characterized as the ratio between the thickness ofthe lower step metal film and the aperture diameter).

[0090] The apertures 206 in opaque film 204 may be formed using amilling technique (for example, focused beam of ions), etching technique(for example, anisotropic plasma etching) combined with e-beam directwrite techniques. Indeed, any technique to form, create and/or produceapertures 206 in opaque film 204, whether now known or later developed,is intended to be within the scope of the present invention.

[0091] The size and shape of apertures 206 determine, to some extent,the number of photons sensed by sensor cells 200 and the maximum spatialfrequency of the measured aerial image. In one embodiment, apertures 206are substantially circular and have a diameter of between approximately50 nm to approximately 200 nm, and preferably between about 75 nm toabout 150 nm. An appropriate size of the diameter of apertures 206 maybe determined using the wavelength of the photons of light 16 and thenumerical aperture of lithographic equipment 10 (typical characterizedas n (i.e., the refractive index of the medium above the sensor or waferin lithographic equipment 10)×sin θ).

[0092] It should be noted that, for those skilled in image processing,it is well known that, assuming the response of the aperture is a simplesummation of all photo energy impinged on it, the aperture will verylikely “behave” like a low pass-filter to the aerial image intensitydistribution. Assuming a square aperture of size d by d, the first zeroin the spatial frequency pass band may be at 1/d. If the aperture is around aperture with diameter d, the first zero in the spatial frequencypass band may be at 1.22/d. To preserve a substantially amount of, orall spatial information, 1/d (for square aperture) or 1.22/d (for roundaperture) should be higher than the maximum spatial frequency existingin the intensity distribution of the aerial image.

[0093] The discussion below may be applicable to round and/or squareapertures. Moreover, the first zero in the spatial frequency pass bandwhen the aperture size is d, where d is either the side length of asquare aperture, or 1/d is the diameter of a round aperture.

[0094] It should be further noted that a response function, which may bean acceptable approximation, is a summation of all the energy impingingon the inlet of the aperture 206. More sophisticated response functionswill likely require detailed calibration and/or implementation ofsuitable computer simulations tools. Once such computer simulation toolis “TEMPEST” available from UC Berkeley.

[0095] Regardless of illumination, partial coherence, and/or ReticleEnhancement Techniques (for example, Optical Proximity Correction (OPC),and Phase-Shift Masks (PSM)) on masks, the maximum spatial frequency inthe light intensity distribution on wafer plane may be characterized as2×NA/λ, where NA is the Numerical Aperture of the stepper projectionoptics, and λ is the wavelength used in the imaging. Therefore, in oneembodiment, the aperture size that preserves all or substantially allspatial frequency, may be characterized as:

1/d>2×NA/λ— or— d<λ/(2×NA)

[0096] In one embodiment, where lithographic equipment 10 includes an NAof 0.75 and employs a wavelength of 193 nm, the size of apertures 206may be smaller than 128 nm. In those instances where lithographicequipment 10 includes an NA of 0.65 employing a wavelength of 248 nm,the size of apertures 206 may be smaller than 190 nm.

[0097] In certain instances, the smaller the size of aperture 206, thehigher spatial frequencies can be preserved; in contrast, the smallerthe size of apertures 206, the less light or radiation passes to sensorcells 200. An aperture size smaller than λ/3 may severely reduce thelight that can pass through, and aperture size smaller than λ/6 mayeffectively block the light passage. Therefore, in one embodiment, theaperture size is equal to or greater than λ/2. As such, for 193 nmwavelength, the aperture size may be 90 nm or greater; and for 248 nmwavelength, the aperture size may be 120 nm or greater.

[0098] Thus, in at least one embodiment, where the dimension size ofactive areas 202 of sensor cells 200 are in the order of a few μm×a fewμm (for example, 2 μm×5 μm), and where a spatial resolution of betweenabout 75 nm to about 150 nm may be desired, required or advantageous,patterned film 204 (for example, tungsten, aluminum, or silicon) may beemployed to limit or restrict the exposed active areas of sensor cells200 a _(x)-200 h _(x) (x=1 to 8) thereby enhancing the spatialresolution of sensor cells 200 a _(x)-200 h _(x) (x=1 to 8). A spatialresolution of between about 75 nm and about 150 nm may be sufficient toproperly, accurately and/or adequately characterize, measure, collect,sense and/or detect the aerial image of mask 26 as projected at thewafer plane.

[0099] It should be noted that in one embodiment the sidewalls ofapertures 206 may be shaped to provide, among other things, an enhancedresponse. After dry etching or ion milling, the sidewall shape may beclose to vertical. By shaping the walls of apertures 206, the shape canbe modified to have tilted angles, which may enhance the amount ofradiation that passes through apertures 206 for the oblique rays. Suchshaping may be by isotropic etching or other suitable technique.

[0100] The dimensions of sensor array 106 may depend, to some extent, onsize of the aerial image to be measured, the technique employed tocapture that image, the data collection time, and/or any the spatialconstraints of sensor array 106 due to constraints imposed by imagesensor unit 102 and/or lithographic equipment 10. In one embodiment,sensor array 106 may be the same, substantially the same or about thesame size as the aerial image to be measured. In this way, the datacollection time is minimized relative to a sensor array of substantiallysmaller size while the footprint of the sensor array is not too large.For example, where the aerial image at the wafer plane is about 26 mm×32mm, sensor array 106 may be about 27 mm×33 mm.

[0101] With reference to FIGS. 7A and 7B, in at least one embodiment,the surface of sensor array 106 (or sensor cells 200) may be coated witha transparent medium 205 having predetermined refractive index “n”. Inone embodiment, the refractive index is equal to or substantially equalto that of photoresist. In this way, a coating may: (1) reduce thewavelength to λ/n within apertures 206, and hence potentially increasethe light energy pass ratio by the aperture; (2) enhance the refractionto oblique rays to make the direction of oblique rays more “straightdown” toward apertures 206, and hence improve the passing ratio ofoblique rays; (3) emulate the refraction effect of resist on aerialimage, and thereby cause the sensed aerial image to be a closerapproximation of the aerial image “inside” resist; and/or (4) increasethe effective ratio between the aperture size and the wavelength (due tothe reduced wavelength inside the aperture), which may reduce theaperture response function's variation due to aperture process variation(for example, the variation in size between the apertures). In thisembodiment, transparent medium 205 may be deposited or grown afterformation of opaque film 204.

[0102] It should be noted that transparent medium 205 may be grown ordeposited in apertures 206 rather than over all or substantially all ofthe surface of sensor cells 200 or sensor array 106. In this embodiment,transparent medium 205 may be deposited or grown before or afterformation of opaque film 204.

[0103] It may be desirable, or in certain circumstances, advantageous,to enhance the photo-reception or photon efficiency of sensor cells 200.With reference to FIG. 8A, in one embodiment, a photo or photondetection enhancement material 208 a _(x)-208 h _(x) (x=1 to 8) (forexample, a photo or photon sensitive semiconductor material) may bedeposited, grown and/or formed within aperture 206 a _(x)-206 h _(x)(x=1 to 8), respectively, to enhance the ability or capacity of sensorarray 106 to measure, sample, sense and/or detect incident photons orenergy at a given wavelength (for example, λ_(o)). Thus, in thisembodiment, detection enhancement material 208 a _(x)-208 h _(x)enhances the ability or capacity of active areas 202 a _(x)-202 h _(x)(x=1 to 8) to measure, sample, sense and/or detect incident radiationand thereby improve the ability or capacity of sensor array 106 tocharacterize, measure, collect, sense and/or detect the aerial image ofmask 26 as projected at the wafer plane.

[0104] With continued reference to FIG. 8A, the detection enhancementmaterial 208 a _(x)-208 h _(x) (x=1 to 8) may be deposited, grown and/orformed before and/or after formation or patterning of apertures 206 a_(x)-206 h _(x) (x=1 to 8). The detection enhancement material 208 a_(x)-208 h _(x) (x=1 to 8) may also be deposited, grown and/or formedbefore and/or after deposition, growth or formation of opaque film 204.One example for the detection enhancement material 208 may be thesemiconductor material used in the photo-sensitive area 202, so thatphotons can be converted to electrons before they travel through theaperture.

[0105] With reference to FIG. 8B, in another embodiment, sensor array106 may include sensor cells 200 a _(x)-200 h _(x) (x=1 to 8) that areinsensitive (or relatively insensitive) to the wavelength of the energyemployed by the photolithographic equipment 10 to expose the waferduring integrated circuit fabrication (for example, photons or light atwavelength λ_(o)). The sensor array 106 may also includephoton-conversion material 210 a _(x)-210 h _(x) (x=1 to 8), forexample, a “lumigen” or “lumogen” material, selectively disposed onactive areas 202 a _(x)-202 h _(x) (x=1 to 8) of sensor cells 200 a_(x)-200 h _(x) (x=1 to 8), respectively, to convert the energy into aform that is measurable by sensor cells 200 a _(x)-200 h _(x) (x=1 to8). In this way, the active areas 202 are effectively limited or reducedproportionally with the footprint of photon-conversion material 210.Accordingly, the photon-conversion material 210 enhances the spatialresolution of each sensor cell 200.

[0106] For example, where sensor array 106 is employed in lithographicequipment 10 that utilizes ultraviolet light, for example, to projectthe aerial image at the wafer plane, sensor array 106 may include sensorcells 200 a _(x)-200 h _(x) (x=1 to 8) that are insensitive toultraviolet light, but measure, sample, sense, detect or collect photonsin the visible light spectrum. The photon-conversion material 210 a_(x)-210 h _(x) (x=1 to 8) may be selectively patterned and disposed onactive areas 202 a _(x)-202 h _(x) (x=1 to 8) of sensor cells 200 a_(x)-200 h _(x) (x=1 to 8), respectively, to convert ultraviolet lightincident on photon-conversion material 210 a _(x)-210 h _(x) (x=1 to 8)to visible light energy (or other wavelength of light that is sensitiveto the sensor cells). Thus, the exposed active areas of sensor cells 200a _(x)-200 h _(x) (x=1 to 8) are effectively limited or restricted whichthereby enhances the spatial resolution of sensor cells 200 a _(x)-200 h_(x) (x=1 to 8).

[0107] The photon-conversion material 210 a _(x)-210 h _(x) (x=1 to 8)may be selectively patterned and disposed on or over active areas 202 a_(x)-202 h _(x) (x=1 to 8) to provide an “effective” active area ofabout 75 nm to about 150 nm. As mentioned above, a spatial resolution ofabout 75 nm to about 150 nm may be sufficient to properly, accuratelyand/or adequately characterize or detect the aerial image of mask 26 asprojected at the wafer plane.

[0108] With reference to FIG. 9A, in another embodiment, opaque film 204(of FIGS. 6A and 6B) is used, in conjunction with photon-conversionmaterial 210 (of FIG. 8B) to enhance the resolution of sensor sells 200a _(x)-200 h _(x) (x=1 to 8) of sensor array 106. In this regard, opaquefilm 204 and photon-conversion material 210 are disposed on or over atleast active areas 202 a _(x)-202 h _(x) (x=1 to 8) of sensor cells 200a _(x)-200 h _(x) (x=1 to 8), respectively. The active areas 202 a_(x)-202 h _(x) (x=1 to 8) of sensor cells 200 a _(x)-200 h _(x) (x=1 to8), respectively, measure, sample, sense, detect and/or are responsiveto visible light. The photon-conversion material 210 converts energy ofa given wavelength to visible light, as described above with respect toFIG. 8A. The opaque film 204 overlies or covers selected portions of thephoton-conversion material 210 and active areas 202 a _(x)-202 h _(x)(x=1 to 8) and forms apertures 206 a _(x)-206 h _(x) (x=1 to 8) in thesame manner as described above with respect to FIGS. 5, 6A and 6B. Assuch, opaque film 204 limits or restricts the photons (of a givenwavelength) incident on the exposed photon-conversion material 210which, in turn, limits or restricts conversion of the incident photonsto visible light to those portions of active areas 202 a _(x)-202 h _(x)(x=1 to 8) that are incident or contiguous to the photon-conversionmaterial 210 exposed via apertures 206 a _(x)-206 h _(x) (x=1 to 8).

[0109] The embodiment of FIG. 9A may provide the advantage that thephotons are still at short ultraviolet wavelength when they pass throughapertures 206 of opaque film 204. As such, the scattering effect of thephotons caused by the apertures of opaque film 204 may be less severewhich may facilitate more photons to pass or travel through theapertures 206, be converted by photon-conversion material 210, andsampled, sensed, detected and/or measured by those portions of activeareas 202 that are contiguous with photon-conversion material 210 thatis exposed by apertures 206 a _(x)-206 h.

[0110] With reference to FIG. 9B, in another embodiment, similar to theembodiments illustrated in FIGS. 7A and 7B, the opaque film 204 andphoton-conversion material 210 of FIG. 9A is coated with transparentmedium 205 having a refractive index “n”. In one embodiment, therefractive index of transparent medium 205 is equal to or substantiallyequal to that of photoresist. As mentioned above, such a coating may:(1) reduce the wavelength to λ/n within apertures 206; (2) enhance therefraction to oblique rays to make the direction of oblique rays more“straight down” toward apertures 206; (3) emulate the refraction effectof resist on aerial image; and (4) reduce the aperture responsevariation between apertures. The transparent medium 205 may be depositedor grown after formation of opaque film 204.

[0111] With reference to FIGS. 9C and 9D, photon-conversion material 210may also be disposed in apertures 206 a _(x)-206 h _(x) (x=1 to 8) offilm 204. For example, photon-conversion material 210 may be deposited,grown and/or formed over sensor cells 200. Thereafter, film 204 may bedeposited, grown and/or formed over or in photo-conversion material 210and apertures 206 a _(x)-206 h _(x) (x=1 to 8) (see, for example, FIG.9C). Alternatively, photon-conversion material 210 may be deposited,grown and/or formed within apertures 206 a _(x)-206 h _(x) (x=1 to 8) ofopaque film 204 (see, for example, FIG. 9D). As described above, thephoton-conversion material 210 coverts photons of a given wavelength toanother wavelength which may be sensed, detected, measured or sampled byselected portions of active areas 202 a _(x)-202 h _(x) (x=1 to 8)defined, for example, by apertures 206 a _(x)-206 h _(x) (x=1 to 8),respectively.

[0112] It should be noted that, in another embodiment, photon-conversionmaterial 210 may overlie or be disposed on opaque film 204. In thisregard, photon-conversion material 210 is deposited, grown and/or formedover opaque film 204. In operation, the photon-conversion material 210coverts photons of a given wavelength to energy of another wavelength(for example, visible light) which is then provided to selected portionsof active areas 202 a _(x)-202 h _(x) (x=1 to 8) defined by apertures206 a _(x)-206 h _(x) (x=1 to 8), respectively. In this way, the spatialresolution of sensor cells 200 is enhanced, limited and/or reduced

[0113] Moreover, with reference to FIG. 10, in another embodiment, aphoto or photon detection enhancement material 208 a _(x)-208 h _(x)(x=1 to 8) (for example, a photo or photon sensitive semiconductormaterial) may be deposited, grown and/or formed within aperture 206 a_(x)-206 h _(x) (x=1 to 8), respectively, to enhance the efficiency ofthe active areas 202 a _(x)-202 h _(x) (x=1 to 8) to receive and detectthe visible light.

[0114] In another embodiment, sensor cells 200 having “effective”portions of active areas 202 of a suitable size may be fabricated bymodifying the electro-gate structure of the CCD cells, CMOS sensorcells, photo diodes, or other light sensing device. In this regard, inone aspect of this embodiment, an aperture in sensor cell 200 may beformed by selectively removing or eliminating certain material overlyingthe active area of sensor cell 200. For example, with reference to FIG.11A, an aperture in sensor cells 200 may be created by removing aportion of the layer(s) that obstructs, absorbs and/or impedes, forexample, ultraviolet light. In this way, when implemented in anultraviolet light based lithographic equipment, sensor cells 200measure, collect, sense and/or detect photons having a wavelength in theultraviolet region via the aperture overlaying the active areas ofsensor cells 200. This technique and structure may be implemented inlithographic equipment or systems employing other wavelengths orlithographic techniques. Thus, the technique and structure of thisembodiment provides enhanced resolution of sensor cells 200 byselectively creating and/or forming an aperture in a certain portion ofthe layer(s) that obstructs, absorbs or impedes photons of a wavelengthto be measured by sensor cells 200.

[0115] It should be noted that the apertures in the electro-gatestructure may be created after fabrication of the sensor cells (forexample, by ion milling or e-beam lithography), or during itsfabrication by modifying the mask used to create the electro-gate of thesensor cells. It should be further noted that in this embodiment, thelayer(s) in which the aperture is formed may be considered to be thesame as opaque film 204 discussed above.

[0116] In yet another embodiment, sensor cells 200 having “effective”portions of active areas 202 of a suitable size may be fabricated byincorporating opaque film 204 (including apertures 206) within themultiple layers of CCD cells, CMOS sensor cells, photo diodes, or otherlight sensing device. In this regard, opaque film 204 is integrated intosensor cells 200 and disposed between a protective outer surface ofsensor cells 200 and active areas 202 of sensor cells 200. For example,with reference to FIG. 11B, opaque film 204 is disposed between activearea 202 and a protective layer of silicon dioxide (SiO₂) or siliconnitride (Si₃N₄). In this embodiment, opaque film 204 may be any materialthat may be deposited, grown and/or formed in sensor cells 200, andpatterned, milled, shaped and/or etched such that active areas 202receive, measure, collect photons from a smaller, limited and/orrestricted area or region (substantially or proportionally equal to thearea of apertures 206 relative to the entire active area 202). Thus, thetechnique and structure of this embodiment provides enhanced resolutionof sensor cells 200 by integrating an additional layer, i.e., opaquefilm 204 (having apertures 206), in sensor cell 200.

[0117] It should be further noted that there are many techniques andmaterials (and, as a result, structures created thereby) for enhancingthe spatial resolution of sensor cells 200. Moreover, there are manytechniques and permutations of depositing, growing, milling and/orforming the various layers of sensor 200, opaque film 204, apertures206, detection enhancement material 208, and/or photon-conversionmaterial 210. For example, with reference to FIG. 11C, detectionenhancement material 208 and/or photon-conversion material 210 may bedisposed in or near apertures 206 (via, for example, CVD, PECVD orimplantation techniques) of the sensor cells illustrated in FIGS. 11A or11B. Thus, all techniques and materials, and permutations thereof, thatenhance the spatial resolution of active areas 202 of sensor cells 200,whether now known or later developed, are intended to be within thescope of the present invention.

[0118] Moreover, in one embodiment, the sensor cells 200 of FIGS. 11A-Cmay also include transparent material 205 (having the predeterminedrefractive index “n”) disposed over apertures 206 and active areas 202of sensor cell 200. In another embodiment, the film 204 includes a stepconfiguration around the proximity of apertures 206 as illustrated inFIGS. 6B and 7B. The discussions of sensor array 200 of FIGS. 6A, 6B, 7Aand 7B are fully applicable to this aspect of the present invention.However, for the sake of brevity, those discussions will not berepeated.

[0119] With reference to FIGS. 3A, 3B and 3C, as mentioned above, imagesensor unit 102 may also include controller 108, a source of electricalpower, for example, batteries 110, data compression circuitry 112,transmitter circuitry 114, transmitter/receiver circuitry 116, memory120 and connector 122. The controller 108, data compression circuitry112, transmitter circuitry 114, transmitter/receiver circuitry 116,and/or memory 120 may be incorporated on image sensor unit 102 asdiscrete components or may be integrated in substrate 118 (via VLSI orLSI integration techniques), or a combination thereof (for example,sensor array 106 is integrated in substrate 118 and transmittercircuitry 114 are discrete components). In those circumstances where theelectronics of image sensor unit 102 is comprised of discrete componentsit may be advantageous to employ surface mount technologies, unpackageddie and/or cavities, holes or cut-outs in substrate 118 in order tofurther minimize the profile of sensor unit 102.

[0120] Further, in those circumstances where the electronics of imagesensor unit 102 is comprised of discrete components, sensor unit 102 maybe a circuit board-like structure which has the same or substantiallythe same two-dimensional shape as a wafer. In these embodiments, thehybrid-type sensor wafer may be thicker than an actual product wafer,where surface topography of substrate 118 and sensor unit 102 may not beflat or uniform. In one preferred embodiment, however the thicknessrange of sensor 102 is within the acceptable parameters to be handled bylithographic equipment 10.

[0121] The substrate 118 may be a blank silicon wafer, or be made ofsome other material (e.g., aluminum carbide) suitable to be placed on astepper stage. The discrete components may be disposed onto thesubstrate, and interconnected through patterned metal wires on or in thesubstrate or via bonding wires.

[0122] For example, where sensor 102 is used in conjunction withlithographic equipment 10 (for example, a stepper), the: (1) totalweight of sensor unit 102 preferably should be within a range from anactual wafer, hence be able to be handled by the stage control systemand wafer handling systems; and (2) the thickness of sensor unit 102preferably should be within a range from actual wafers so that the stagemay handle to keep the appropriate focus on the surface of sensor array106. Of particular interest is a configuration that maintains sensorarray 106, and potentially a focus area around it, at the same thicknessas a typical wafer, so that the auto-focus system of lithographicequipment 10 (if any) may achieve focus on the surface of sensor array106 without adjusting the Z-height of the wafer stage. It should benoted that areas outside sensor array 106 may not need to have the samethickness of a typical wafer. Preferably, however, there is enough“clearance” relative to the working distance of stepper optics and otherwafer handling systems to permit automatic handling and/or placement onchuck 22 with no or minimal modification to lithographic equipment 10.

[0123] In those instances where sensor array 106 is a discrete device(or is manufactured in or on substrate 118), the “flatness” of thesurface of sensor array 106 may be an important consideration, becausefor the sensor array to measure, sense, sample and detect usefulinformation, all cells in the array need to lie within the depth offocus of lithographic equipment 10. For example, where lithographicequipment 10 is a stepper having a numerical aperture NA and using aradiation source of wavelength λ, the depth of focus may becharacterized as λ/(NA)². As such, for 193 nm stepper with NA=0.75, thedepth of focus is 340 nm and, it may be preferable to maintain a surfaceflatness of sensor array 106 smaller than the depth of focus, forexample, within 100 nm.

[0124] It should be noted that in those situations where lithographicequipment 10 employs UV light, it is preferable to employ a sensor array106 that is UV-stable (i.e., does not emit any significant contaminationdetrimental to lithographic equipment 10 (or portions thereof) underillumination. Moreover, it may be advantageous to passivate sensor unit102 and/or sensor array 106, for example, with a coating of an inertmaterial, to enhance the structural and/or performance stability ofsensor unit 102 and/or sensor array 106. The sensor array 106 and/orsensor unit 102 may also be coated with anti-reflection materials toreduce reflecting light back into lithographic equipment 10 (forexample, the optical sub-system of lithographic equipment 10).

[0125] When using anti-reflection (AR) costing on the sensor array 106,the coating may cover the entire sensor array 106, or may only cover thearea outside the apertures 206. The AR coating may be applied before orafter the apertures 206 are processed. When the AR is coated before theapertures are processed, the aperture processing may be drilling oretching the apertures through both the AR coating and the blocking layer204. To reduce the total aperture aspect ratio for the benefitsdescribed before, it is maybe advantageous to recess the AR coating fromthe aperture, so that the AR layer's thickness does not contribute tothe total aperture sidewall height. When fabricating the AR recess, itis similar to create the second layer of 204 as described above inconjunction with FIG. 6B and 7B. The recessed AR may be the secondlayer, or coincide with the second layer of 204, or be the third layer.

[0126] With continued reference to FIGS. 3A, 3B and 3C, in certainembodiments, controller 108 coordinates the operations and/or functionsof the various electronics on or in image sensor unit 102. In thisregard, controller 108 may coordinate the sampling of data by sensorarray 106 with the exposure and movement operations by lithographicequipment 10. The controller 108 may also coordinate the operationand/or timing of the data communication, data collection and storage,clocking, synchronization, and testing.

[0127] In particular, controller 108 may be employed to: (1) interpretand execute the received commands, for example, from the input wired orwireless channel, (2) generate and manage clock signals for otherelectronics on sensor unit 102, (3) synchronize job start and operationsof sensor array 106, compression circuitry 112, and/or wired or wirelesstransmission (for example, Tx 114 and/or Tx/Rx 116); (4) monitor theoperating conditions of sensor array 106 and/or sensor unit 102 (forexample, temperature, power, and local current); (5) perform, implementand/or coordinate self-testing of the electronics on sensor unit 102(for example, sensor cells 200 of sensor array 106); (6) store andprovide calibration and/or implementation data of the electronics onsensor unit 102 (for example, sensor array 106 and/or compressioncircuitry 112); (7) store and provide operations information to theelectronics on sensor unit 102, including commands and operatingconditions; and (8) perform and schedule testing of sensor unit 102(based, for example, on historical information relating to testing andtest results). Indeed, such testing may include statistical processcontrol functions for sensor unit 102 and provide relevant warning orpreventative maintenance requests to processor/controller 104. Further,based on monitored information, sensor unit 102 may provide warningmessages or emergency shutdown safety functions.

[0128] The controller 108 may also have resident memory to storefirmware, calibration data and/or sensor history. The controller, likethe other electronics of sensor unit 102, may be implemented via FPGA,DSP, microprocessor or microcontroller, and/or an ASIC.

[0129] In certain embodiments, image sensor unit 102 may include asource of electrical power, for example, batteries 110 (for example, theembodiments of FIGS. 3A and 3B). In this regard, batteries 110 may be aprimary source of electrical power and thereby may provide all orsubstantially all of the power requirements of image sensor unit 102.The batteries 110 may be rechargeable (after one or several aerial imagecapture routines). For example, batteries 110 may include lithium-ionbased batteries and/or polymer based batteries. Where the invention isemployed in a clean room environment, for example, batteries 110 shouldbe stable and durable to prevent leakage.

[0130] The batteries 110 may be customized to accommodate the shape andprofile constraints of the wafer-shaped embodiments of sensor unit 102.Further, batteries 110 may be disposed in cavities, holes or cut-outs insubstrate 118 in order to minimize the profile of image sensor unit 102.Thus, image sensor unit 102 of these embodiments may be implemented inlithographic equipment 10 as a self-contained and/or self-sufficientsensing device without the need of electrical connection to lithographicequipment 10 or elsewhere.

[0131] In certain embodiments, the source of power may be provided toimage sensor unit 102 using connector 122 (see, for example, FIGS. 3Band 3C). In this way, some or all of the electronics may be powered byan external power source, which may facilitate use of higher powerconsumption components that may offer features not offered by lowerpower consumption components (for example, speed).

[0132] It should be noted that in those instances where image sensorunit 102 is incorporated or integrated into chuck 22 of lithographicequipment 10, power may be provided by lithographic equipment 10. Inthis embodiment, as discussed in detail below, there may be no need toemploy batteries 110, unless, for example, as an auxiliary or back-uppower source, in the event of a power failure to sensor unit 102.

[0133] With reference to FIGS. 3A, 3B and 3C, image sensor unit 102 mayalso include data compression circuitry 112 to compress the image data(for example, 8 bits per pixel to represent a pixel's gray scale value)collected by sensor array 106 before transmission or communication toprocessor/controller 104. The data compression circuitry 112 may reducethe bandwidth requirements of the data communications. In thoseinstances where wireless transmission is employed to provide data toprocessor/controller 104, compression of the data (via circuitry 112)may significantly reduce the bandwidth requirements of transmittercircuitry 114 and transmitter/receiver circuitry 116 and, as such,provide the data representative of the aerial image toprocessor/controller 104 in a more efficient, less time consumingmanner. Compression of the data may also reduce the power consumption bysuch wireless transmission circuitry.

[0134] The data compression circuitry 112 may be in or on FPGA,microprocessors, DSP and/or ASIC(s). As mentioned above, in thosecircumstances where data compression circuitry 112 is a discretecomponent, it may be advantageous to employ surface mount technologies,unpackaged die and/or cavities, holes or cutouts in substrate 118 inorder to minimize the profile of sensor unit 102 (if necessary).

[0135] It is preferred to use lossless data compression techniques. Inthis way, no information is lost in compression.

[0136] For an optical image of integrated circuit pattern, a 2:1lossless compression ratio may be suitable. Many compression algorithmsare suitable and are well known to those skilled in the art.

[0137] In one embodiment, to improve the compression ratio, the data maybe arranged to improve the correlation between image data. In thisregard, the image usually gives good correlation between neighboringpixels. In those instances where the data is provided to compressioncircuitry 112 in its sequence of availability, for example, sub-frame bysub-frame. A sub-frame refers to the array of output of an array ofsensor cells, when the sensor array is positioned at a fixed locationrelative to the aerial image. The sub-frame embodiments will bediscussed in more detail below, for example, in conjunction with FIGS.12A-C, 13A-E, and 14A-C), the inter-pixel correction may be minimum. Assuch, it may be desirable to setup a buffer (using, for example, memory120) to store or maintain several (for example, 2, 4, or 8) sub-framesbefore providing the data to the compression engine of compressioncircuitry 112. The data sequence may be re-arranged in the buffer sothat the values of neighboring pixels (i.e., from neighboringsub-frames) are juxtaposed or stored in another predeterminedarrangement that improves the compression ratio.

[0138] In another embodiment, image processing is performed on the dataprior to providing the data to the compression engine of compressioncircuitry 112. For example, data which is representative of the aerialimage measured, sensed, detected and/or sampled by sensor cells 200 maybe processed to reduce, minimize or eliminate the noise content withinthe data. This may be accomplished using data from neighboring pixels.Thereafter, the pre-processed data may be provided to the compressionengine of compression circuitry 112 and the compressed data may betransmitted or communicated to processor/controller 104. By performingsome or all of the noise processing functions to image processingcircuitry on image sensor unit 102, the compression ratio may improve.

[0139] In those instances where the functions and/or operations ofprocessor/controller 104 are integrated on image sensor unit 102 or inlithographic equipment 10, there may be no advantage to compress thedata which is representative of the aerial image before transmission. Inthis regard, data may be provided to the transmission circuitry via aninternal bus in or on image sensor unit 102, or via connector 122directly to lithographic equipment 10 or to processor/controller 104. Inthis circumstance, it may be advantageous to employ a high-speed and/orwide data bus as a mechanism for data transmission from image sensorunit 102 to the appropriate circuitry of processor/controller 104.

[0140] With continued reference to FIGS. 3A, 3B and 3C, image sensorunit 102 may employ wired, wireless and/or optical data transmission. Inthose instances where wireless transmission is implemented as atechnique to provide some or all data and commands to/from image sensorunit 102 (see, for example, FIGS. 3A and 3B), image sensor unit 102 mayinclude transmitter circuitry 114 and/or transmitter/receiver circuitry116. In this way, image sensor unit 102 may be implemented inlithographic equipment 10 in the same manner as a product wafer.Moreover, where all data and commands are provided via wirelesstechniques (see, for example, FIG. 3A), image sensor unit 102 may beimplemented in lithographic equipment 10 as a self-contained and/orself-sufficient unit without the need of electrical connection of anykind to/from lithographic equipment 10.

[0141] In those embodiments where image sensor unit 102 employs wiredand/or optical data transmission, connector 122 may be disposed onsubstrate 118 to provide a mechanism for external communication. Theconnector 122 may be an electrical connector that includes signal, powerand ground pins or contacts, where signals are transmitted usingproprietary or non-proprietary protocols. The connector 122 may also bean optical connector (for example, an optical transmitter/receiver) thatcommunicates signals using well-known protocols.

[0142] It should be noted that there are many wireless technologies thatmay be implemented. For example, Radio Frequency (RF) based wirelesscommunication technologies may be more suitable than other free-spacetechniques because such techniques provide high data transfer rates butdo not require the presence of a line of sight. Other suitable wirelesstechnologies include, for example, infrared and free-space opticalcommunications. Indeed, all such wireless communication techniques,whether now known or later developed, are intended to be within thescope of the present invention.

[0143] In one embodiment, wireless local area network technologies, forexample, 802.11a/b/g, may be employed for the RF based wirelesstransmission. Indeed, in one embodiment, multiple channels of 802.11a/g(each channel supports 54 Mbps raw data rate) may be implemented tooutput data (for example, sensor data), and one channel of 802.11b (eachchannel supports 11 Mbps data rate) may be implemented to input data(for example, commands and/or configuration data). Moreover,implementing a configuration where there is short distance betweenantennas, the power consumption of these multiple channels may besignificantly reduced without adversely affecting the communicationquality.

[0144] Further, when using 802.11 technologies, or other general-purposecommunication techniques, the communication protocols may be slightlymodified to reduce the amount of overhead data, and hence increase theeffective payload bit rate of the communication channels. For example,the “top-layer” in 802.11, namely TCP (Transmission Control Protocol)may be eliminated, which would eliminate the header data associated withit. In this way, the amount of overhead data is reduced.

[0145] In those instances where RF wireless is implemented, an antennamay be placed on or close/near to lithographic equipment 10 (forexample, on or close/near to the door of the stepper) to communicatewith an antenna disposed on image sensor unit 102. The distance betweenthe two antennas may be very short, in the order of a few feet, which isthe distance between the wafer stage and the stepper door. As such, thewireless transmission data rate may be high, in the range of hundreds ofmega bits per second.

[0146] After the data is received at the antenna outside lithographicequipment 10, the data may be converted to other digital data formats,and provided to processor/controller 104 using wired, wireless and/oroptical transmission (for example, wired Giga-bit Ethernettransmission). Implementing a wired or optical approach may minimize the“contamination” of the RF bands outside lithographic equipment 10. Inthis way, processor/controller 104 may be located some distance from thelithographic equipment 10, and even outside the clean room in thoseinstances where the lithographic equipment 10 is employed for integratedcircuit manufacture, for example.

[0147] In many embodiments, the output bandwidth requirement (i.e.,transmission of image data from sensor unit 102) is likely to be largerthan the input bandwidth (i.e., receipt of commands from, for example,processor/controller 104). In one embodiment, the output bandwidthsupports a “real-time” image capturing data rate. In this embodiment,the image data is provided to the processor/controller 104 in real time(i.e., while sensor unit 102 is measuring, sampling and/or capturingimage data). The input signals, in this embodiment, may include commandsfrom processor/controller 104, for example, start and configurationinformation.

[0148] In particular, in at least one example, the data rate may be inthe order of 200 Million pixels per second for a 3 cm×3 cm lithographicstepper printing field, a sensor array 106 having 100 Million sensorcells 200 each having a 3 μm pixel size, and an exposure rate of ½seconds per exposure. Further, where 8 bits are used to represent eachpixel's gray level, the data rate is in the range of 1.6 G bits/second.

[0149] It should be noted that, as mentioned above, the transmissiondata rate may be reduced using a number of techniques including, forexample, data compression, noise reduction and buffering (elastic orotherwise). Assuming 2:1 data compression, a desired output bandwidthmay be in the range of 800 M bits per second (Mbps). In short, there aremany techniques to reduce the bandwidth requirement; accordingly, allsuch techniques, whether now known or later developed, are intended tobe within the scope of the present invention.

[0150] It should be further noted that in some embodiments, a ForwardError Correction (FEC) technique may be employed to reduce the amount ofpotential re-transmission due to error or loss, if any, during wirelesstransmission.

[0151] In certain embodiment, the image data measured, sampled, and/orcaptured by sensor unit 102 may not be transmitted toprocessor/controller 104 in real time (i.e., while the image is beingmeasured and the sensor unit 102 is still on the stepper stage), but maybe stored in memory 120 (FIGS. 3B and 3C). After the data collection andafter the sensor unit 102 is unloaded from the stepper stage, the imagedata may be downloaded to processor/controller 104 through wired,wireless, and/or optical communications, for example, using connector122.

[0152] With reference to FIG. 3D, in another embodiment, sensor array106 may be disposed in, or integral with chuck 22 of lithographicequipment 10. In this embodiment, image sensor unit 102 need not beloaded into the lithographic equipment but may be positioned in thewafer plane (for example, by chuck 22) during inspection,characterization and/or evaluation of photomask 26 and/or the opticalcomponents of lithographic equipment 10. As mentioned above, electricalpower may be provided to the components of image sensor unit 102 fromlithographic equipment 10. Moreover, image sensor unit 102 mayreceive/provided data and commands using wired, wireless or opticalcommunications.

[0153] The image sensor unit 102 of this embodiment may also include acontroller, a source of electrical power, data compression circuitry,communications circuitry, memory and/or connector, as described abovewith respect to FIGS. 3A, 3B and 3C. This circuitry (and the functionsperformed thereby) may be integrated into components and/or circuitry inlithographic equipment 10 or external thereto. For example, thefunctions and operations performed by the controller of image sensorunit 102, as described above, may be undertaken or performed by acontroller resident in lithographic equipment 10 or by a separate,dedicated device.

[0154] In addition, power to the components of image sensor unit 102(for example, sensor array 106) may be provided by lithographicequipment 10 or by a dedicated power supply. Further, image sensor unit102 may include separate communications circuitry, for example,components to implement wireless, wired and/or optical techniques; orimage sensor unit 102 may employ the communication circuitry typicallypresent in lithographic equipment 10.

[0155] In operation, image sensor unit 102 measures, collects, sensesand/or detects an aerial image produced or generated by the interactionbetween photomask 26 and lithographic equipment 10 (whether in situ ornot). In one embodiment, the image data which is representative of theaerial image is collected by repeatedly exposing sensor array 106 with aspatial shift (relative to the aerial image) between exposures. Aftereach exposure, the sensor array 106 provides a sparsely sampledsub-image frame, which may be referred to as a sub-image or sub-frame indiscussions below. The sub-images are interleaved to generate, create,provide and/or build-up the full-field image or to extract desiredinformation directly without reconstructing the aerial image.

[0156] In particular, with reference to FIGS. 12A-C and 13A-E, in oneembodiment, sensor array 106 is located in a first location relative tothe aerial image projected on the wafer plane (See, for example, FIG.13A). While in the first location, the aerial image is measured, sensed,detected and/or sampled by sensor cell 200 ax-200 gx (x=1 to 7) and thedata representative of the sample or measured values are provided to theother circuitry on image sensor unit 102 (for example, data compressioncircuitry 112, transmitter 114, and/or memory 120) for processing,transmission and/or storage.

[0157] The sensor array 106 is then moved to a second location, viachuck 22, a distance Δx from the first location (see, for example, FIG.13B). In one embodiment, Δx is substantially equal to the diameter orsize of apertures 206. In another embodiment, the spatial shift issubstantially equal to the “effective” active area 202 of sensor cell200. Where the diameter or size of apertures 206 is about 75 nm, thespatial shift of a distance Δx may be about 50 nm to about 75 nm.

[0158] While in the second location, the aerial image is again measured,sensed, detected and/or sampled by sensor cells 200 ax-200 gx (x=1 to7). The measured or sampled values (data representative of the aerialimage) are again provided to the other circuitry on image sensor unit102 (for example, data compression circuitry 112, transmitter 114,and/or memory 120) for processing, transmission and/or storage.

[0159] The sensor array 106 is then moved to a third location (adistance Δx from the second location) and the aerial image is againmeasured, sensed, detected and/or sampled by sensor cells 200 ax-200 gx(x=1 to 7) (see, for example, FIG. 13C). As before, the measured orsampled values are provided to the other circuitry on image sensor unit102 for processing and/or storage.

[0160] In one embodiment, this data collecting or sampling processcontinues in the x-direction until a portion of the aerial image that iscollected by a given sensor cell 200 is contiguous (or substantiallycontiguous) with the first location collected or sampled by an adjacentlateral sensor cell (See, FIG. 13D). Thereafter, sensor array 106 (viachuck 22) is moved in the y-direction a distance Δy from the previouslocation (See, FIG. 13E). In one embodiment, Δy is substantially equalto the diameter or size of apertures 206. In another embodiment, thespatial shift Δy is substantially equal to the “effective” active area202 of sensor cell 200. Where the diameter or size of apertures 206 isabout 75 nm, the spatial shift of a distance Δy may be about 50 nm toabout 75 nm.

[0161] While re-positioned a distance Δy, the aerial image is againmeasured, sensed, detected and/or sampled by sensor cells 200 a _(x)-200g _(x) (x=1 to 7). The measured or sampled values are provided to theother circuitry on image sensor unit 102 for processing, transmissionand/or storage. The sensor array 106 is then moved to a distance −Δx andthe aerial image is again measured, sensed, detected and/or sampled bysensor cells 200 a _(x)-200 g _(x) (x=1 to 7). This process is continuedin the x and y directions until the aerial image (or portion thereof) ismeasured, sensed, detected, collected and/or sampled. That is, thesensor array 106 may be moved, positioned and/or re-positioned in an xand y direction until the entire, or selected portion of the aerialimage has been measured, sensed, detected, collected and/or sampled.(See, for example, FIG. 12B) Thereafter (or simultaneously), the datameasured, collected, sensed, detected and/or sampled at each location isprocessed and compiled into a full image (or portion thereof) that isrepresentative of the aerial image formed by the stepper onto the waferplane.

[0162] It should be noted that the data may also be collected in avector fashion, for example, using absolute x and y coordinates toeffectively guide the exposure to a particular portion of the aerialimage. The data is collected, measured, sensed, detected and/or sampledin the same manner as described above. The aerial image may be processedand compiled in the same manner as well. In this way, particular area(s)of a mask may be examined in situ or certain locations or areas of theoptical system of lithographic equipment 10 may be examined.

[0163] The size of the spatial shift may be a “pixel” of the finalcaptured aerial image. In one embodiment, the spatial shift is equal toor substantially equal to the “effective” active area 202 of sensor cell200. In another embodiment, the spatial shift is equal to orsubstantially equal to the size of aperture 206. In either embodiment,the data is collected or sampled in a raster fashion, for example, using“small” relative movement in an x and/or y direction. In this way, eachexposure provides a sub-image or sub-frame; thereafter, all thesub-images are interleaved to build up the full-field (or partial-field)aerial image.

[0164] Since each sub-frame is captured by different exposures of theimaging field by lithographic equipment 10, there may be slightmisalignment errors or a given amount of alignment offset between thesub-frames. These errors or offset considerations may be caused bynon-perfect stage positioning repeatability. In a preferred embodiment,these errors or offsets may be calibrated by aligning each sub-frameimage to database image, and, as such, be compensated in subsequentimage processing steps. This sub-frame-to-database alignment step iscalled sub-frame alignment. Indeed, in one embodiment, historical dataor the statistics of the sub-frame offset represents the stage'spositioning repeatability, and hence may be used as information forstepper monitoring.

[0165] When the stage positioning is highly repeatability, for example,significantly smaller than of the pixel size (less than 20% of the pixelsize), it may be possible to perform inter-sub-frame alignment withoutthe database, to calibrate out the stage positioning offset errorsbetween exposures. This step maybe accomplished through the optimizationof the image and/or edge smoothness by fine adjustment of the relativeposition between the sub-frames. This alignment procedure, withoutdatabase image, is called inter-sub-frame alignment. Like sub-framealignment, the results of inter-sub-frame alignment may also be used asinformation for stepper monitoring.

[0166] In one embodiment, the image collecting and/or sampling techniqueemploys the highly precise spatial positioning and/or movement of chuck22 to collect and/or sample the aerial image. In another embodiment,chuck 22 and sensor 102 may remain stationary and the optics oflithographic equipment 10 and mask 26 may move.

[0167] As mentioned above, the sampled or measured values of sensorcells 200 ax-200 gx (x=1 to 7) may be “pixels” of the aerial image. Inone embodiment, the “pixel” resolution may be equal (within 10%tolerance) or substantially equal (i.e., within 25% tolerance) to thedimensions of apertures 206 ax-206 gx (x=1 to 7). The size and/or shapeof apertures 206 may be adjusted, designed or modified to accommodate anumber of considerations including, for example, the features of theaerial image (critical dimensions), the fabrication techniques of thesensor array 106, the image acquisition time, the desired or necessaryimage resolution, and/or the wavelength of light 16. While illustratedas substantially circular in FIGS. 13A-E, the apertures 206 may besubstantially square, rectangular, triangular or oval. Indeed, any shapemay be implemented.

[0168] Moreover, the aperture may be shaped to match with certain testmask design pattern. For example, in certain application of the presentinvention such as focus analysis, the resolution along one direction maybe most significant/important. Under that circumstance, the dimension ofthe aperture in the other direction may be larger to achieve higherphoton passing rate.

[0169] It should be noted that the spatial shift Δx and/or Δy may begreater than or less than the diameter or size of apertures 206 and/orthe “effective” active area 202 of sensor cell 200. In this regard,where the spatial shift Δx and/or Δy is less than the diameter or sizeof apertures 206 and/or the “effective” active area 202 of sensor cell200, system 100 may be over-sampling the aerial image to, for example,provide a highly precise aerial image. Indeed, the over-sampled data maybe used to confirm or verify the accuracy of surrounding data, oreliminate or minimize the need for data interpolation or extrapolation,or ensure that no spatial information is lost (i.e., the image betweensampled pixels can be exactly interpolated (this is called the Nyquisttheorem).

[0170] As described above, regardless of illumination, partialcoherence, or reticle enhancement techniques on masks, in certainembodiments, the maximum spatial frequency in the light intensitydistribution on wafer plane is 2×NA/λ, where NA is the NumericalAperture of the stepper projection optics, and λ is the wavelength usedin the imaging. Employing this relationship, the Nyquist sampling ratefor aerial image in a stepper is 4×NA/λ. As such, the pixel size may beat p=λ/(4×NA) or smaller. For wavelength of 193 nm, and NA=0.75, thepixel size p may be 64 nm or smaller. For wavelength of 248 nm, andNA=0.65, the pixel size p may be 95 nm or smaller.

[0171] The pixel size may be equal or substantially equal to the shiftbetween sub-frames. Therefore, with reference to FIG. 12B, when thedistance between sensor cells 200 (the cell size may be equal to orsubstantially equal to as the distance between neighboring apertures206) is C, in order to collect all spatial information, there will be atotal of C/p sub-frames along x-direction, and all of them repeated C/ptimes along y-direction. As such, to collect or build-up a continuousfull frame image, (C/p)² sub-frames should be collected. This determinesthe throughput of the full-field continuous aerial image capturing.

[0172] For example, where the size of sensor cell 200 is 9 μm, and apixel size of 75 nm is employed, the total number of sub-frames requiredwill be (9000/75)²=14400. Under this circumstance, where a stepper takes0.5 second to make one exposure, the total time required will be 7200seconds, which is 2 hours.

[0173] In certain embodiments, the sampled areas are not continuous. Forexample, when the aerial image is used to map out the linewidthvariation across the field (also called CD or critical dimension),collecting, sampling, measuring and/or sensing an entire full-fieldimage may not be necessary or useful. In this regard, it may besufficient to have blocks of small images distributed across the field,with each block sampled at Nyquist rate. With reference to FIG. 14A, inone embodiment, an array of image blocks where each block is sampled atNyquist, and the block array covers the entire field. This samplingstrategy may be called “block-mode sampling”. In this embodiment, acontinuous full field image is not generated, collected, sampled, sensed(compare FIG. 12B).

[0174] Using block-mode sampling, the image capturing time may besignificantly reduced. For example, using the same considerations asdescribed above, assuming a 2.25 μm×2.25 μm area for each block is to becollected, the number of sub-frames is reduced to (2250/75)²=900, fromthe original 14400. This is a 15× reduction in data collection time.

[0175] Further, block-mode sampling may facilitate using a sensor array106 having sensor cells 200 that are larger (for example, 10 μm, 20 μm,or larger). In addition, this sampling technique may permit use of thepixel decimation mode that is available in most CCD chips operations.The pixel decimation mode bins multiple neighboring cells into a singlecell (for example, 1×2 or 2×2). This effectively enlarges the cell sizeand reduces the cell numbers, which thereby reduces the amount of datato be transmitted.

[0176] There are many applications that may employ block-mode sampling,for example: (1) full-field CD metrology; (2) full-field stepper lensaberration calibration; (3) full-field stepper printing field distortioncalibration; and (4) full-field process window analysis. All suchapplications, whether now known or later developed, are intended to bewithin the scope of the present invention.

[0177] It should be noted that when implementing block-mode sampling,sporadic non-functional cells in sensor array 106 may not affect thefunctionality of the system 100 because the sampling techniqueinherently produces a loss of certain blocks; however, the across fieldstatistics are still collected using the functional cells.

[0178] With reference to FIG. 14B, in another embodiment, the number ofsub-frames may be further reduced to two one-dimensional samples. Forexample, where the application of the system 100 is to calibrate thefocus plane location for x and y directional lines using a speciallydesigned grating mask, the one-dimensional sampling along both x and ymay suffice. The one-dimensional sampling may be either continuous ornot continuous. Under this circumstance, and using the sameconsiderations as described above, in a non-continuous application, thetotal number of sub-frames needed will be only (2250/75)+(2250/75)=60.In contrast, in the continuous application, the number of sub-frameswill be (9000/75)+(9000/75)=240.

[0179] With reference to FIG. 14C, in another embodiment,one-dimensional sampling may be applied to any direction. In thisregard, a 90-degree sampling technique is illustrated. Otherone-dimensional sampling techniques may also be implemented (forexample, 0-degree and 45-degree).

[0180] Sensor array 106 may be larger or smaller (or substantiallylarger or smaller) than the aerial image to be measured. In thosecircumstances where sensor array 106 is larger, certain sensor cells 200may be located outside or beyond the projected aerial image (in eitheran x, y or x-y direction) and as such, a portion of the data measured bycertain sensor cells 200, a certain portion of the data measured bysensor cells 200, or all of the data measured by certain cells 200 maybe discarded because that data is not representative of, or related orpertinent to the aerial image. While some data may be discarded orunnecessary, where sensor array 106 is larger than the aerial image tobe measured, any constraints, limitations or requirements of x-yalignment of the aerial image on the wafer plane may be reduced oreliminated altogether.

[0181] In those circumstances where sensor array 106 is smaller than theaerial image to be measured, chuck 22 may be positioned andrepositioned, in a tile or block like manner, to collect the entireimage (see, for example, FIGS. 15A-B, 19 and 20). The dimensions of eachtile or block may be equal to or substantially equal to the dimensionsof sensor array 106. When positioned at a first location, sensor array106 may collect an image data set, as described in detail above, of theaerial image projected at that position. Thereafter processor/controller104 may process the image data set measured, collected, sensed and/ordetected at each position to generate or create an aerial image for eachresponsive position. The processor/controller 104, in turn, may thenconnect or combine the pieces or portions of the aerial image in amosaic-like fashion, to produce a larger portion, or the entire, aerialimage.

[0182] In particular, with reference to FIGS. 15A and 15B, while sensorarray 106 is appropriately positioned, a first frame (i.e., Frame 1) issampled and collected, as described above with respect to FIGS. 12A-C,13A-E and/or 14A-C. Thereafter, sensor array 106 may be re-positioned tosample and collect a second frame (i.e., Frame 2), again as describedwith respect to FIGS. 12A-C, 13A-E and/or 14A-C. The other frames (ifany) may be collected in the same manner. The aerial image is generatedusing the data sampled and collected for each frame.

[0183] The present invention may be implemented to detect or inspect forcontamination, for example, mask contamination or optical lenscontamination. In this regard, the contamination refers to those thatcreate large-area but small-magnitude intensity change in the finalaerial image. In one embodiment, the aerial image may be collected,sampled and/or measured, below the Nyquist rate. With reference to FIG.16, the aperture size may be larger than the aperture size determinedabove. In this embodiment, the appropriate aperture size to be used maydepend on the size of the contamination defects that need to becaptured. It should be noted that both the aperture size (graphicallyrepresented by the dots) and pixel size (the spacing between dots) areenlarged.

[0184] In another embodiment, system 100 may collect image data usingsensor unit 106 by scanning sensor array 106 across the image field.With reference to FIG. 17, in one embodiment, a complete (or partial)aerial image is collected, measured, sampled or built-up by scanningsensor cells 200 in one direction at a small angle to the arrayx-coordinate such that all y-adjacent pixels on the image are covered byone scan.

[0185] It should be noted that other scanning or imaging techniquesusing image sensor unit 106 may be implemented to collect, measure,sample and/or build-up a complete (or partial) aerial image. All suchtechniques, whether now known or later developed, are intended to bewithin the scope of the present invention.

[0186] As mentioned above, the data which is representative of theaerial image (measured, collected, sampled, captured by sensor cells200) is processed by processor/controller 104 to generate the aerialimage projected on the wafer plane. In one embodiment, the aerial imageis formed by the same imaging path of lithographic equipment 10(including mask 26) that is used to print product wafers. By monitoringthe actual aerial image, the present invention enables the end-to-end,close-loop process optimization. That is, optimization from design andfabrication of mask 26, to lithographic equipment 10 selection, toset-up of lithographic equipment 10.

[0187] When employed as a lithography inspection system 100, the presentinvention may automatically compensate for the defects that areanticipated or expected to occur during processing, i.e., mask,illumination, optics, contamination, and interactions there between.Obtaining the aerial image projected on the wafer plane also enablesanalysis of the impact on printability and yield of a defect detectedand/or sensed, thereby allowing full lithography process integritycontrol.

[0188] The system of the present invention may also facilitate isolatingsources of errors or problems caused or produced during the lithographyprocess. For example, the impact of a given defect in mask 26 may bedetermined by substituting a mask having the same pattern thereon forthe “defective” mask. Since lithographic equipment 10 remains constant,if the defect is again detected or measured in the aerial image, thatdefect may be attributed to the optics of lithographic equipment 10.However, if the defect is not detected or measured, that defect may beattributed to the “defective” mask. In this way, the sources of errorsor problems may be isolated to the mask or lithographic equipment 10.

[0189] It should be noted that the image may also isolate imagingproblems from resist development and substrate etching, providingcritical or useful information for process development. Moreover, thepresent invention may also be used, in conjunction with softwareemulating the resist processing, to predict the developed resist image.Indeed, the present invention may be used, in conjunction with directSEM inspection of the developed resist image, to verify the aboveemulation. In fact, the aerial image captured using the presentinvention, in conjunction with SEM inspection of the final developedresist image, may be further used to extract accurate resist models.

[0190] In addition to the capabilities of aerial image monitoring anddefect inspection discussed above, the present invention(s) may also beimplemented in wafer pattern metrology. In this regard, the presentinvention(s) may facilitate full-field, non-destructive, in-situ, andreal circuit pattern or critical dimension measurements by comparing thesensed image to the mask pattern design database. In certain instances,it may be necessary to convert the polygon data within the designdatabase to intensity data. Alternatively, the data representative ofthe information sampled, measured, detected, and/or sensed may beconverted to data that permits comparison to the polygon data maintainedin the design database.

[0191] The inventions described herein may also be used in steppercalibration (for example, aberration and field distortion calibration).Information captured, obtained and/or calculated during imageprocessing, may also be used for stepper monitoring. For example, thesub-frame alignment offset may be used to monitor stage positioningrepeatability, and the image quality may be used to monitor stepper lenscontamination and lens drift.

[0192] It should be noted that one task of the algorithm(s) implementedby processor/controller 104 may be image processing to reconstruct theoriginal un-filtered aerial image. Such image processing may involvedeconvolution or other techniques of two-dimensional image processing.

[0193] As mentioned above, in certain embodiments, the image processingalgorithms may be implemented before a full image is received, sampled,measured and/or captured by sensor unit 102. For example,processor/controller 104 may begin processing data received from sensorunit 102 before a full image is provided and/or sampled, measured and/orcaptured by sensor unit 102. In this regard, processor/controller 104may initiate and implement decompression, data structure setup,sub-frame alignment, inter-sub-frame alignment, and noise reductionalgorithms.

[0194] As mentioned above, in certain applications, a database image isemployed to maintain the theoretical aerial image based on the maskpattern design and the ideal optics. As such, in certain embodiments,because the computation of the database image is extremely intensive, itmay be advantageous to compute the database image “offline” and storedin the image computer in certain data structure that is easy to beretrieved and assembled. This offline database computation may usestorage space, but the reduction in real time computation may presenteconomical architectural advantages for system 100. Furthermore, whenthe computed database is stored, it may be stored in compressed format(lossless compression) to minimize or reduce the necessary storagespace. Such compression may be very effective (for example, better than5:1) for database images, since there is no noise in the database image.

[0195] For some applications, for example, CD metrology, processingwindow analysis, and mask design verification, the aerial image may bereconstructed. Yet for some other applications, the actual aerial imageneed not be reconstructed, but the sensor image itself may suffice, forexample, image field placement distortion, and mask defect inspection.When performing Die-to-Die (D:D) mask inspection, direct comparisonbetween the sensor images from multiple dice may be suitable. ForDie-to-Database (D:DB) mask inspection, the database may be directlyrendered to its theoretical image under “ideal” stepper and sensorconfiguration, and, as such, allow the D:DB inspection by comparing thesensor image to that “theoretical” database image.

[0196] In those instances where mask 26 includes OPC or PSMdecoration/features, the database used in D:DB mask inspection may beeither the one with the decoration, or the one without decoration. Thedatabase with decoration is typically used to make the mask, and isexpected to be consistent with mask 26. The database without decorationis generally known as the “design-target” (i.e., the target on-waferimage which combines the effect from OPC/PSM decoration and the imagingpath of the stepper). When using database with decoration, the databaserendering should fully consider the stepper optics effect. When usingthe design-target database, the stepper imaging effect is alreadyembedded and hence need not be computed again.

[0197] In one embodiment, the D:DB inspection method may include thecomparisons of the: (1) decorated image to the sensor image orreconstructed aerial image (using sensor unit 102) and (2) design-targetto the sensor image or reconstructed aerial image. In this embodiment,the decoration is inspected or verified through using the sensor imageor reconstructed aerial image. For example, where there is an error inthe decoration (for example, the OPC software made a mistake indecoration), and the mask is made “correctly” that is, the decoration isaccurately produced according to the OPC software, the optical imageproduced by that mask will show that there is no defect when comparedwith the decorated database. However, the optical image produced by thatmask will reveal a defect when compared with the design-target, andhence an OPC decoration error may be detected.

[0198] Thus, the present invention may be employed, for example, in: (1)stepper development, including lens aberration calibration, focuscalibration, field distortion calibration, illumination calibration; (2)stepper qualification, including separation of stepper errors fromresist development and etching errors, aerial image quality assessment;(3) stepper monitoring, including contamination monitoring, stagepositioning repeatability monitoring, stepper aberration monitoring,illumination drift monitoring; (4) process development, includingprocess window analysis, resist model extraction; (5) full fieldmetrology, including linewidth (CD) measurement, contact energymeasurement; (6) mask inspection, including D:D and D:DB inspection,mask contamination inspection; (7) mask design verification, includingOPC and PSM decoration verification; (8) process optimization, includingmask-specific adjustment and centering of process window, selection ofoptimization of mask-stepper pairing, adjustment of stepper parameters(for example, illumination filter, partial coherence, pupil filter, andso on) using mask-specific aerial images; (9) design to processoptimization, including aerial image analysis to capture potentialyield-loss hot spots, Design-Rule-Checking (DRC) on aerial images; (10)design verification and optimization, for example, extraction ofelectrical performance information, such as resistance, current,voltage, timing, noise, power, etc from a set of aerial images, and thesubsequent use of such extract electrical performance data of thecircuit for design optimization; and (11) failure analysis, for example,use the aerial images to analyze the potential failure mechanism (e.g.,overlay-induce shorts) if the chip does not work as designed. Indeed,all applications of the present invention(s), whether now know or laterdeveloped, are intended to come within the scope of the presentinvention(s).

[0199] It should be noted that lithographic equipment 10 and/orprocessor/controller 104 may employ system 100, in conjunction withcontrol software of lithographic equipment 10, to enable rapid and/orreal-time optimization. In one embodiment, the system of the presentinvention may be combined with a specially designed mask to monitor thelithographic equipment 10, for example, optical aberration, fielddistortion, and illumination. The system 100 may then provide thereal-time feedback to control software of lithographic equipment 10 (forexample the stepper) to implement system modifications to minimize theaberrations and field distortion, and improve the illuminationuniformity across the imaging field.

[0200] In another embodiment, the system of the present invention maydirectly sense the aerial image of the production mask, and compute theadjustment to the stepper settings to optimize the aerial image quality.This may facilitate mask-stepper-combination-specific optimization. Forexample, where the mask has a slow spatial CD variation, the system ofthe present invention may measure and detect that CD variation andfeedback the suggested changes to the stepper illumination settingsacross the field to compensate the mask's CD non-uniformity. For certainareas that have a smaller CD than expected or permitted, the stepper mayadd more illumination dose to compensate for such an imperfection. Sucha feature may significantly enhance the parametric yield of thelithography process, since it is well known in the art that CDuniformity directly translates into the speed at which the IC chip canbe run.

[0201] In another example, the images captured using the system of thepresent invention may be used to adjust and center, for eachmask-stepper combination, the optimal process window, allowing morerobust and higher yield IC fabrication.

[0202] The system of the present invention may also be implemented tooptimize for specific product masks. For example, where the locations ofcritical patterns are known, apertures 206 may be located to be morerapidly and easily aligned with the critical patterns. Indeed, the shapeof apertures 206 may also be selected to better suit such anapplication. This may significantly improve the speed of imagecapturing, since there is no need to capture the sub-frames that do notcontain the critical patterns.

[0203] Further, the customized aperture shape may significantly improvethe performance of the monitoring. For example, the customized apertureshape may significantly improve monitoring gate linewidth CD in, forexample, a microprocessor. Thus, in those instances where the locationsof the critical-path transistors are known, apertures 206 may belocated, positioned or aligned with the location of the gates of thosetransistors in order to more rapidly and accurately monitor the CD ofsuch integrated circuits. Moreover, the shape of apertures 206 may alsomatch the gates.

[0204] Yet another application in analyzing the aerial image is tocombine images from multiple exposures. In this regard, for some PSMdesigns, multiple masks are made to expose the same layer. For example,in one implementation of alternating-PSM (see, for example, U.S. Pat.No. 6,228,539), a first mask has multiple phases, and a second mask(called trim mask) has only a single phase, and both masks are exposedon the same layer to create the desired effect in the photo-resist. Thesystem of the present invention may record the sensed aerial image fromthe first mask, then the sensed aerial image from the second mask isadded to the first aerial image, and hence obtain the combined effect ofthe two masks in the photo-resist. The combined image may then be usedto compare with the design-target image, for mask inspection, or forverifying the PSM design and decorations.

[0205] In the case of multiple masks, the individually captured aerialimages of each mask may be further used to optimize the combination inactual process, for example, relative dose. The individually capturedaerial images may also be used to analyze the process tolerance, forexample, the overlay tolerance between the two masks, and where the mostdefect-prone spots are located. This inter-layer analysis may not onlybe applied to the multiple-mask-single-layer case as described above,but also between different adjacent layers on the circuit, for example,to analyze and optimize the interlayer overlay tolerance between thepoly-gate layer and the contact layer using the aerial images from thosetwo layers.

[0206] Yet another potential application of aerial images captured frommultiple exposures is to analyze and inspect the phase information inPSM. For example, multiple aerial images can be captured at differentfocus planes for a PSM, and the images from these multiple focus planescan be used to extract the phase shift amount for each phase-shiftregion on the PSM, and be compared and verified again design.

[0207] The system of the present invention may also be used inStatistical Process Control (“SPC”). In this regard, the system 100monitors the stepper performance every time it is used, and hence tracksthe history of the performance of the stepper to provide, for example,preventive maintenance alerts.

[0208] The system of the present invention may also be used to inspectand optimize certain maskless lithography technologies. “Masklesslithography technology” generally refers to the lithography techniquesthat do not use a mask, rather the patterns are written on the waferdirectly by the lithography tool, e.g., an electron-beam direct-writelithography tool. One category of maskless lithography technologiesincludes a “programmable mask”, i.e., the pattern is still defined by a“mask”, but the mask is programmable using the database and directlyresides in a lithography tool, e.g. a micro-mirror array where eachmirror can turn on and off of a pixel.

[0209] In the maskless technology that uses a programmable mask, therewill be no mask to inspect. One way to assure the quality of theprogrammed mask and the quality of the printed pattern is to directlymeasure, analyze, and/or inspect the aerial image generated or producedby this maskless lithography system using the sensor unit 102 and thetechniques described above. Under this circumstance, the aerial imageanalysis may be directly feedback to the maskless lithography system, toadjust and optimize the programming of the programmable mask, andthereby enable direct lithography quality optimization.

[0210] As mentioned above, in one aspect the present invention may be(or be implemented in) an imaging system that generates or produces thesame or substantially the same aerial image (or producing, sampling,collecting and/or detecting information relating to), with the same orsubstantially the same spatial resolution, as certain lithographicequipment (for example a particular stepper system having a given set ofparameters, features or attributes). In this regard, the imaging systememulates that lithographic equipment. Thus, in this embodiment, theimaging system includes a precision mechanical movement stage having thesame or substantially the same mechanical precision and controllabilityas lithographic equipment. The imaging system may be employed as astandalone aerial image monitoring tool that may be used in thereviewing of the aerial image of a mask under the predetermined opticsub-system.

[0211] The stepper-like tool (stepper, mini-stepper, or other imagingoptics) together with system (or portions thereof described above withrespect to image sensor unit 102 and processor/controller 104 may be astand-alone aerial image inspection tool. This stand-alone tool may beemployed, for example, to perform mask inspection and/or mask defectreview. In this embodiment, the aerial image is sensed directly, athigh-NA and at the same magnification as on the product wafer. As such,not only is the wavelength and partial coherence matched (orsubstantially matched), but also the NA is matched (or substantially)with actual steppers, eliminating the potential deviation from actualaerial image when vector field in EM wave is considered. In this regard,the fidelity of the aerial image may be improved.

[0212] It should be noted that the discussions of, for example, imagesensor unit 102 and processor/controller 104, are fully applicable tothis aspect of the present invention. For the sake of brevity, thosediscussions will not be repeated.

[0213] There are many inventions described and illustrated herein. Whilecertain embodiments, features, attributes and advantages of theinventions have been described and illustrated, it should be understoodthat many other, as well as different and/or similar embodiments,features, materials, attributes, structures and advantages of thepresent inventions, are apparent from the description, illustration andclaims. As such, the embodiments, features, materials, attributes,structures and advantages of the inventions described and illustratedherein are not exhaustive and it should be understood that such other,similar, as well as different, embodiments, features, materials,attributes, structures and advantages of the present inventions arewithin the scope of the present invention.

[0214] It should be noted that while the present invention(s) isdescribed in the context of measuring, inspecting, characterizing and/orevaluating optical lithographic equipment, methods, and/or materialsused therewith, for example, photomasks, the present invention may beused to measure, inspect, characterize and/or evaluate other opticalsystems. Indeed, the image sensor unit described herein may be used tomeasure, inspect, characterize and/or evaluate microscopes andtelescopes. Or, the present invention can be combined with a microscopeor telescope optics and a precise mechanic stage, to realizesub-optical-wavelength resolution in image sensing. As such, any opticalsystem, method and/or material used therewith, whether now known orlater developed, are intended to be within the scope of the presentinvention.

[0215] Moreover, it should be noted that while the present invention(s)is described generally in the context of integrated circuit fabrication,the present invention(s) may be implemented in processes to manufactureother devices, components and/or systems including, for example,photomasks, hard disk drives, magnetic thin-film heads for hard diskdrives, flat panel displays, and printed circuit board. Indeed, thepresent invention(s) may be employed in the fabrication of any devices,components and/or systems, whether now known or later developed, thatmay benefit (in whole or in part) from the present invention(s).

[0216] For example, in other applications or industries, substrate 118may take a different form factor and may be made from differentmaterials. For example, in photomask manufacturing, using laser exposureor other optical imaging exposure techniques, the substrate may be amask blank (glass or quartz), or other material with the same shape of aphotomask (which maybe a square plate of 5 or 6 inches each side, withthickness of a few millimeters). In Flat Panel Display manufacturing,the substrate may be a high quality glass plate of a predetermined shapeand size. In hard disk drive manufacturing, the substrate is alsowafer-like, but made from different materials. In printed circuit board(PCB) manufacturing, the substrate is a circuit board. It should benoted that the present invention(s) may be implemented using the given(different) substrate form-factors and/or materials of the particularapplication in which the invention is implemented. Such substrates mayinclude one, some or all of the functionalities and capabilitiesdescribed herein. Indeed, other functionalities and capabilities may bedesired depending upon the particular application in which the inventionis implemented.

[0217] It should be further noted that there are many techniques andmaterials (and, as a result, structures created thereby) for enhancingthe spatial resolution and/or sensitivity of sensor cells 200. Indeed,there are many techniques and permutations of depositing, growing and/orforming opaque film 204, apertures 206, detection enhancement material208 and/or photon-conversion material 210. All techniques and materials,and permutations thereof, that enhance, limit or restrict the spatialresolution of active areas 202 of sensor cells 200, whether now known orlater developed, are intended to be within the scope of the presentinvention.

[0218] Further, while apertures 206 have been generally illustrated ashaving substantially vertical sidewalls, the sidewalls may include atapered edge. Such sidewalls may be formed using a variety offabrication techniques. For example, apertures 206 may be formed using acombination of anisotropic and isotropic etching techniques that form atapering at the edges of the sidewalls after the relatively verticaletch is completed.

[0219] Indeed, another technique of limiting, restricting or enhancingthe spatial resolution or sensitivity of sensor cells 200 of sensorarray 106 is to employ anomalously high transmission of light or photonsin arrayed apertures in a film (see, for example “Extraordinary OpticalTransmission through Sub-wavelength Hole Arrays”, T. W. Ebbesen et al.Nature 391, 667, (1998) and T. J. Kim et al. “Control of OpticalTransmission through Metals Perforated with Sub-wavelength Hole Arrays”,Optics Let. 24 256 (1999), the contents of which are hereby incorporatedby reference). Where the film includes more than one aperture per activearea/sensor cell, the system 100 may employ deconvolution or other imageprocessing techniques to appropriately characterize, sense, detect,measure and/or sample the aerial image of mask 26 as projected at thewafer plane. Indeed, more than one aperture may be avoided by usingblind (partially milled or etched) apertures or other surfacemodifications.

[0220] In addition, there are many techniques implemented by thestructures of the inventive sensor unit. For example, communicationsbetween sensor unit 102 and processor/controller 104 may be viaelectronic, optical, wired or wireless. As such, suitable circuitry (forexample, transmitters, receivers, connectors, data compressioncircuitry, controller and memory) may be implemented on or in sensorunit 102 to accommodate the various means of communication (see, forexample, FIG. 3A (primarily wireless), FIG. 3B (wireless, optical and/orwired) and/or FIG. 3C (primarily optical and/or wired)). Indeed, allforms of communication, whether now known or later developed areintended to fall within the scope of the present invention.

[0221] Moreover, the communications of the data representative of theaerial image may be during data collection and/or after data collection.Such communication may be while sensor unit 102 resides in thelithographic equipment (for example, during data collection) or whilethe sensor unit is external to the lithographic equipment (for example,after data collection). Indeed, sensor unit 102 may include sufficientmemory (for example, DRAM, SRAM, or flash) to store some or all of thedata representative of the aerial image in order to increase theflexibility of data transmission (i.e., when such data is transmittedand how such data is transmitted). Indeed, in those instances wheresensor unit 102 is transmitting data during collection, memory 120 maybe employed as a buffer for such data communications/transmission.

[0222] In addition, it should be noted that although sensor cells 200 ofsensor array 106 have been described above to be arranged in an arrayconfiguration, other configurations may be suitable. Moreover, thenumber of sensor cells 200 employed to comprise sensor array 106 may beselected based on various considerations, including, for example, datacollection time, the size of the aerial image at the wafer plane, thespatial resolution of the data collection, and/or the spatial resolutionof active areas 202 of sensor cells 200.

[0223] It should be further noted that the dimensions of sensor array106 may depend on a number of considerations including, for example, thenumber of sensor cells 200 employed, the size of the aerial image at thewafer plane, the spatial resolution of the data collection, the spatialresolution of active areas 202 of sensor cells 200, constraints based onthe data collection time, and/or data collection techniques. In oneembodiment, the sensor array 106 is approximately 27 mm×33 mm and theaerial image (at the wafer plane) is about 26 mm×32 mm.

[0224] As mentioned above, sensor cells 200 may be CCDs, CMOS sensordevices, photodiode devices or the like. Moreover, sensor cells 200 maybe a combination of such devices. Indeed, any device(s) that measures,senses, detects and/or samples light, whether known or later developed,is intended to fall within the scope of the present invention.

[0225] Moreover, sensor array 106 may be comprised of a plurality ofsub-sensor arrays. For example, with reference to FIGS. 18A and 18B,sensor array 106 is comprised of sub-sensor arrays 106 a-d. Eachsub-sensor array 106 a-d may be comprised of sensor cells 200 asillustrated in FIGS. 4-10 and/or described above. Such a configurationmay provide an advantage of image data collection, and the speedthereof, since arrays 106 a-d are smaller than the combined sensor array106 and associated interface circuitry may collect and compile theinformation more rapidly, thereby reducing the inspection time. Further,it also reduces the requirement on the size of a single sensor arrayunit.

[0226]FIG. 19 illustrates one embodiment where multiple sub-sensorarrays are combined to construct a larger image sensor array. Thesub-sensor arrays 106 a-f in FIG. 19 are arranged in such a way thatsensor array 106 (which includes 106 a-f) is moved, relative to thefield, four times to cover the area between the chips. In oneembodiment, the operation sequence may be: (1) use the sub-framecapturing, then buildup the image for the first location of each sensorchip; (2) step the array to a previous uncovered area in the imagingfield (represented by a different pattern or shading in FIG. 19; (3)repeat the sub-frame capturing process again to build-up the image inthis new chip array location; (4) move the array, repeat the processuntil all area in the imaging field is covered.

[0227] It should be noted that it is acceptable to have double coverage(i.e., overlap between frames) in the butting areas between the fourlocations of the sensor array. So, the areas covered by different chiparray location do not need to tile up seamlessly.

[0228] Another scanning or tile-up technique, though it uses slightlymore sensor chips (106 a-h), is illustrated in FIG. 20. In thisembodiment, the array is positioned in three different locationsrelative to the imaging field, and hence this embodiment increases theimage-capturing throughput.

[0229] It should also be noted that, for applications that useblock-mode sampling, it may be acceptable to miss or eliminate the imageblocks between the chips that are covered by the sensor readoutcircuitry and bonding pads. In that case, the tiling can be as simple astiling up the sensor chips one to another, like the scheme illustratedin FIG. 18A-B. This configuration/technique allows the use of a singlelocation of the chip array relative to the image, and hence avoid thethroughput hit described in association with FIGS. 19 and 20.

[0230] Finally, it should be further noted that the term “circuit” maymean, among other things, a single component or a multiplicity ofcomponents (whether in integrated circuit form or otherwise), which areactive and/or passive, and which are coupled together to provide orperform a desired function. The term “circuitry” may mean, among otherthings, a circuit (whether integrated or otherwise), a group of suchcircuits, a processor(s), a processor(s) implementing software, or acombination of a circuit (whether integrated or otherwise), a group ofsuch circuits, a processor(s) and/or a processor(s) implementingsoftware. The term “data” may mean, among other things, a current orvoltage signal(s) whether in an analog or a digital form. The term“sample” means, among other things, measure, sense, inspect, detect,capture and/or evaluate. Similarly, the phrase “to sample” or similar,means, for example, to measure, to sense, to inspect, to detect, tocapture, to evaluate, to record, and/or to monitor.

What is claimed is:
 1. An image sensor unit, for use with a highlyprecise moveable platform, the image sensor unit comprising: a substratehaving a wafer-shaped profile; a sensor array, disposed on thewafer-shaped substrate, the sensor array including: a plurality ofsensor cells wherein each sensor cell includes an active area to senselight of a predetermined wavelength that is incident thereon; and afilm, disposed over the active areas of the plurality of sensor cellsand comprised of a material that impedes passage of light of thepredetermined wavelength, wherein the film includes a plurality ofapertures which are arranged such that an aperture of the plurality ofapertures overlies an active area of a corresponding sensor cell toexpose a portion of the active area and wherein light of thepredetermined wavelength is capable of being sensed by the portion ofthe active area that is exposed by the corresponding aperture.
 2. Theimage sensor unit of claim 1 further including a transparent medium,having a predetermined refractive index, disposed on the sensor array.3. The image sensor unit of claim 1 further including communicationscircuitry disposed on the substrate, wherein the communicationscircuitry is coupled to the sensor array.
 4. The image sensor unit ofclaim 1 further including at least one battery, disposed on thewafer-shaped substrate, to provide electrical power to the sensor array.5. The image sensor unit of claim 1 further including at least onebattery, disposed within a cavity in the wafer-shaped substrate, toprovide electrical power to the sensor array.
 6. The image sensor unitof claim 1 further including communications circuitry, disposed on thesubstrate, to provide data to external circuitry using wirelesstechniques.
 7. The image sensor unit of claim 6 wherein thecommunications circuitry provides data using wireless techniques duringcollection of image data by the sensor array.
 8. The image sensor unitof claim 6 further including at least one battery disposed on or withina cavity in the wafer-shaped substrate wherein the battery provideselectrical power to the sensor array and the communications circuitry.9. The image sensor unit of claim 1 wherein the image sensor unitfurther includes: data storage circuitry, coupled to the sensor array,to receive and store the data from the sensor array; data compressioncircuitry, coupled to the data storage circuitry, to compress the data;communication circuitry, coupled to the data compression circuitry, toprovide data to external circuitry; and at least one rechargeablebattery, to provide electrical power to the data storage circuitry, thedata compression circuitry and the communication circuitry.
 10. An imagesensor unit, for use with a highly precise moveable platform, the imagesensor unit comprising: a wafer-shaped substrate; a sensor array,integrated into the substrate, the sensor array including: a pluralityof sensor cells wherein each sensor cell includes an active area tosense light of a predetermined wavelength that is incident thereon; anda film, disposed over the active areas of the plurality of sensor cellsand comprised of a material that impedes passage of light of thepredetermined wavelength, wherein the film includes a plurality ofapertures which are arranged such that an aperture of the plurality ofapertures overlies an active area of a corresponding sensor cell toexpose a portion of the active area and wherein light of thepredetermined wavelength is capable of being sensed by the portion ofthe active area that is exposed by the corresponding aperture.
 11. Theimage sensor unit of claim 10 further including communicationscircuitry, disposed on the substrate, to provide data to externalcircuitry using wireless techniques.
 12. The image sensor unit of claim10 further including at least one battery disposed on or within a cavityin the wafer-shaped substrate wherein the battery provides electricalpower to the sensor array and the communications circuitry.
 13. Theimage sensor unit of claim 10 further including a battery to provideelectrical power to the sensor array.
 14. The image sensor unit of claim10 wherein the image sensor unit further includes: data compressioncircuitry, coupled to the sensor array, to compress the data receivedfrom the sensor array; communication circuitry, coupled to the datacompression circuitry, to provide data to external circuitry; and atleast one rechargeable battery, to provide electrical power to the datacompression circuitry and the communication circuitry.
 15. The imagesensor unit of claim 10 wherein the image sensor unit further includes:data storage circuitry, coupled to the sensor array, to receive andstore data from the sensor array; data compression circuitry, coupled tothe data storage circuitry, to compress the data; communicationcircuitry, coupled to the data compression circuitry, to provide data toexternal circuitry; and at least one rechargeable battery, to provideelectrical power to the data storage circuitry, the data compressioncircuitry and the communication circuitry.
 16. The image sensor unit ofclaim 10 wherein the sensor cells are charge coupled devices, CMOSdevices or photodiodes.
 17. The image sensor unit of claim 10 furtherincluding photon-conversion material, disposed over the sensor array andbetween the film and the plurality of sensors.
 18. The image sensor unitof claim 17 wherein the photon-conversion material is disposed withinthe plurality of apertures of the film.
 19. The image sensor unit ofclaim 17 wherein the photon-conversion material is disposed on thesensor array.
 20. A system to collect image data which is representativeof an aerial image of a mask that is projected on a wafer plane, thesystem comprising: an optical system to produce the image of the mask onthe wafer plane; a moveable platform; an image sensor unit, disposed onthe moveable platform, to collect image data which is representative ofthe aerial image of the mask, the image sensor unit includes: awafer-shaped substrate; a sensor array, disposed on or in thewafer-shaped substrate, wherein the sensor array is located in the waferplane, the sensor array includes: a plurality of sensor cells whereineach sensor cell includes an active area to sense light of apredetermined wavelength that is incident thereon; and a film, disposedover the active areas of the plurality of sensor cells and comprised ofa material that impedes passage of light of the predeterminedwavelength, wherein the film includes a plurality of apertures which arearranged such that an aperture of the plurality of apertures overlies acorresponding active area of a corresponding sensor cell to expose aportion of the active area and wherein light of the predeterminedwavelength is capable of being sensed by the portion of the active areathat is exposed by the corresponding aperture.
 21. The system of claim20 further including communications circuitry, disposed on thesubstrate, to provide image data to a data processing unit usingwireless techniques.
 22. The system of claim 21 wherein thecommunications circuitry provides the data during collection of imagedata by the sensor array.
 23. The system of claim 21 further includingat least one battery disposed on or within a cavity in the wafer-shapedsubstrate wherein the battery provides electrical power to the sensorarray and the communications circuitry.
 24. The system of claim 20wherein the mask is a product-type mask.
 25. The system of claim 20wherein the aerial image produced on the wafer plane is the same orsubstantially the same image produced on a product wafer.
 26. The systemof claim 20 further including: a data processing unit configured toreceive the image data which is representative of the aerial image; andwherein the moveable platform moves in first and second directions to aplurality of discrete locations wherein at each discrete location, thesensor cells sample the light incident on the exposed portion of theactive area and wherein the processing unit uses the data to generate anaerial image.
 27. The system of claim 26 wherein the distance betweenthe plurality of discrete locations in the first direction is less thanor equal to the width of the apertures.
 28. The system of claim 26wherein the distance between the plurality of discrete locations in thesecond direction is less than or equal to the width of the apertures.29. The system of claim 26 wherein the processing unit interleaves theimage data to generate the aerial image.
 30. The system of claim 26wherein the image sensor unit collects data which is representative ofthe aerial image in a raster-type manner.
 31. The system of claim 26wherein the image sensor unit collects image data which isrepresentative of the aerial image in a vector-type manner.
 32. An imagesensor unit which is capable of collecting image data which isrepresentative of an aerial image of a mask that is projected on a waferplane by a lithographic unit, the image sensor unit comprising: a sensorarray, disposed in the moveable platform of the lithographic unitwherein the sensor array is capable of being located in the wafer plane,the sensor array includes: a plurality of sensor cells wherein eachsensor cell includes an active area to sense light of a predeterminedwavelength that is incident thereon; and a film, disposed over theactive areas of the plurality of sensor cells and comprised of amaterial that impedes passage of light of the predetermined wavelength,wherein the film includes a plurality of apertures which are arrangedsuch that an aperture of the plurality of apertures overlies acorresponding active area of a corresponding sensor cell to expose aportion of the active area and wherein light of the predeterminedwavelength is capable of being sensed by the portion of the active areathat is exposed by the corresponding aperture.
 33. The image sensor unitof claim 32 wherein the mask is a product-type mask.
 34. The imagesensor unit of claim 33 wherein the aerial image produced on the waferplane is the same or substantially the same image produced on a productwafer.
 35. The image sensor unit of claim 32 wherein the sensor cellsare charge coupled devices, CMOS devices or photodiodes.
 36. The imagesensor unit of claim 32 wherein the sensor array is capable of movingbetween a plurality of discrete locations in first and second directionswhile disposed on the moveable platform, and wherein the sensor cellssample the light incident on the exposed portion of the active area ateach discrete location.
 37. The image sensor unit of claim 32 whereinthe distance between the plurality of discrete locations in the firstdirection is less than or equal to the width of the apertures
 38. Theimage sensor unit of claim 32 wherein the distance between the pluralityof discrete locations in the second direction is less than or equal tothe width of the apertures
 39. The image sensor unit of claim 32 whereinthe image sensor unit collects data which is representative of theaerial image in a raster-type manner.
 40. The image sensor unit of claim32 wherein the image sensor unit collects image data which isrepresentative of the aerial image in a vector-type manner.